Also, if you have both post-count-based membergroups and non-post-count-based membergroups (There should be abbreivations, for those!), make sure the permissions that they have don't cross-over. A member could be in two groups, you see...?
That's probably not what your problem is. Ricky's covered that. But, I thought it worth mentioning, because a lot of members fall foul of that. (I don't have any post-count groups, because of that.)