I have a forum that has posts not displaying in it in my UBB Version: 7.2 board.
If I do a search for posts that arn't displaying in the forum, i can find/read/edit them.
If I sticky the post, it will display again. If I unsticky it after that, it will again vanish. It's not a matter of the post being old or anything like that. It's a 1 page forum.
At the bottom of a forum list there's a Display Options table. Have you checked that you're not, for example, only showing posts active within the last N months?
BTW, I don't think this was exactly "Admin" related