The Gifted for Leadership blog has a short but interesting interview with Nancy Beach of Willow Creek Community Church about leading volunteers in your church. Key quote:

Volunteers can feel used and sort of spit up almost very easily in church, unfortunately. They can feel as though the only thing that people care about is that they show up and do the task. And I think a lot of good leadership is looking at them as whole people, recognizing that volunteers are going to go through seasons where maybe they have to take a little break or maybe because of something going on in their family or their work life outside of here, they can’t be as big a contributor in terms of their time as they once were. But if you have a long-term view of leading them, you see that there will be ebbs and flows, and you care about the whole person. And if you have a short-term view, you just try to pump people up to do something noble and very time consuming for a bit. And just about anybody can do that, but my question would be: Are those people still going to be with you two, three years from now?

I like her approach--looking at your volunteers with a longer-term vision in mind might help you feel less frustrated when they periodically need to step back from service, and in turn they'll feel less guilty about taking time off every now and then to deal with family or other issues.