From time to time, a volunteer in a position of ongoing responsibility (e.g., patch manager, translation manager, etc.) will become unable to perform the duties of the position. It may be because the job turned out to be more work than he anticipated, or it may be due to completely external factors: marriage, a new baby, a new employer, or whatever.
When a volunteer gets swamped like this, he usually doesn't notice it right away. It happens by slow degrees, and there's no point at which he consciously realizes that he can no longer fulfill the duties of the role. Instead, the rest of the project just doesn't hear much from him for a while. Then there will suddenly be a flurry of activity, as he feels guilty for neglecting the project for so long and sets aside a night to catch up. Then you won't hear from him for a while longer, and then there might or might not be another flurry. But there's rarely an unsolicited formal resignation. The volunteer was doing the job in his spare time, so resigning would mean openly acknowledging to himself that his spare time is permanently reduced. People are often reluctant to do that.
Therefore, it's up to you and the others in the project to notice what's happening—or rather, not happening—and to ask the volunteer what's going on. The inquiry should be friendly and 100% guilt-free. Your purpose is to find out a piece of information, not to make the person feel bad. Generally, the inquiry should be visible to the rest of the project, but if you know of some special reason why a private inquiry would be better, that's fine too. The main reason to do it publicly is so that if the volunteer responds by saying that he won't be able to do the job anymore, there's a context established for your next public post: a request for a new volunteer to fill that role.
Sometimes, a volunteer is unable to do the job he's taken on, but is either unaware or unwilling to admit that fact. Of course, anyone may have trouble at first, especially if the responsibility is complex. However, if someone just isn't working out in the task he's taken on, even after everyone else has given all the help and suggestions they can, then the only solution is for him to step aside and let someone new have a try. And if the person doesn't see this himself, he'll need to be told. There's basically only one way to handle this, I think, but it's a multistep process and each step is important.
First, make sure you're not crazy. Privately talk to others in the project to see if they agree that the problem is as serious as you think it is. Even if you're already positive, this serves the purpose of letting others know that you're considering asking the person to step aside. Usually no one will object to that—they'll just be happy you're taking on the awkward task, so they don't have to!
Next, privately contact the volunteer in question and tell him, kindly but directly, about the problems you see. Be specific, giving as many examples as possible. Make sure to point out how people had tried to help, but that the problems persisted without improving. You should expect this email to take a long time to write, but with this sort of message, if you don't back up what you're saying, you shouldn't say it at all. Say that you would like to find a new volunteer to fill the role, but also point out that there are many other ways to contribute to the project. At this stage, don't say that you've talked to others about it; nobody likes to be told that people were conspiring behind his back.
There are a few different ways things can go after that. The most likely reaction is that he'll agree with you, or at any rate not want to argue, and be willing to step down. In that case, suggest that he make the announcement himself, and then you can follow up with a post seeking a replacement.
Or, he may agree that there have been problems, but ask for a little more time (or for one more chance, in the case of discrete-task roles like release manager). How you react to that is a judgement call, but whatever you do, don't agree to it just because you feel like you can't refuse such a reasonable request. That would prolong the agony, not lessen it. There is often a very good reason to refuse the request, namely, that there have already been plenty of chances, and that's how things got to where they are now. Here's how I put it in a mail to someone who was filling the release manager role but was not really suited for it:
> If you wish to replace me with some one else, I will gracefully > pass on the role to who comes next. I have one request, which > I hope is not unreasonable. I would like to attempt one more > release in an effort to prove myself. I totally understand the desire (been there myself!), but in this case, we shouldn't do the "one more try" thing. This isn't the first or second release, it's the sixth or seventh... And for all of those, I know you've been dissatisfied with the results too (because we've talked about it before). So we've effectively already been down the one-more-try route. Eventually, one of the tries has to be the last one... I think [this past release] should be it.
In the worst case, the volunteer may disagree outright. Then you have to accept that things are going to be awkward and plow ahead anyway. Now is the time to say that you talked to other people about it (but still don't say who until you have their permission, since those conversations were confidential), and that you don't think it's good for the project to continue as things are. Be insistent, but never threatening. Keep in mind that with most roles, the transition really happens the moment someone new starts doing the job, not the moment the old person stops doing it. For example, if the contention is over the role of, say, issue manager, at any point you and other influential people in the project can solicit for a new issue manager. It's not actually necessary that the person who was previously doing it stop doing it, as long as he does not sabotage (deliberately or otherwise) the efforts of the new volunteer.
Which leads to a tempting thought: instead of asking the person to resign, why not just frame it as a matter of getting him some help? Why not just have two issue managers, or patch managers, or whatever the role is?
Although that may sound nice in theory, it is generally not a good idea. What makes the manager roles work—what makes them useful, in fact—is their centralization. Those things that can be done in a decentralized fashion are usually already being done that way. Having two people fill one managerial role introduces communications overhead between those two people, as well as the potential for slippery displacement of responsibility ("I thought you brought the first aid kit!" "Me? No, I thought you brought the first aid kit!"). Of course, there are exceptions. Sometimes two people work extremely well together, or the nature of the role is such that it can easily be spread across multiple people. But these are not likely to be of much use when you see someone flailing in a role he is not suited for. If he'd appreciated the problem in the first place, he would have sought such help before now. In any case, it would be disrespectful to let someone waste time continuing to do a job no one will pay attention to.
The most important factor in asking someone to step down is privacy: giving him the space to make a decision without feeling like others are watching and waiting. I once made the mistake—an obvious mistake, in retrospect—of mailing all three parties at once in order to ask Subversion's release manager to step aside in favor of two other volunteers. I'd already talked to the two new people privately, and knew that they were willing to take on the responsibility. So I thought, naïvely and somewhat insensitively, that I'd save some time and hassle by sending one mail to all of them to initiate the transition. I assumed that the current release manager was already fully aware of the problems and would see the reasonableness of my point immediately.
I was wrong. The current release manager was very offended, and rightly so. It's one thing to be asked to hand off the job; it's another thing to be asked that in front of the people you'll hand it off to. Once I got it through my head why he was offended, I apologized. He eventually did step aside gracefully, and continues to be involved with the project today. But his feelings were hurt, and needless to say, this was not the most auspicious of beginnings for the new volunteers either.