So let's say you have a big job on your forum that needs taking care of, such as installing mods or writing a batch of forum tutorials. And, one of your staff members enthusiastically signs up for the job. Then, later you find out, the person hasn't even started on the job. In these situations, I think it would be nice if the forum staff member, at least updated about the personal situation that was causing the delays, rather than giving no updates at all. For my situation, that would be good, so I could work on the project for myself while I was waiting for their help.
some information would be nice as to why they're doing not their work, but if they don't, then a demotion would be necessary. Choose someone else better suited for the job and actually willing to help out. If you're too busy you may not be able to do the required work.
Surprisingly enough, I run into this sort of situation quite alot. It usually ends up in a situation where I'm doing all of the work in the end anyway, so I've been cutting down on that lately.
It often happens indeed. Situation could change, and so is interest. What we need to do is to follow up and ask the progress every once in a while so we won't be left in the dark without knowing the task is forgotten. It definitely hinders the whole process. That is why communication and teamwork is important.