Not sure how, but I moved a forum out of it's parent category and could no longer see it on the bulletin board.
By resetting the forum's "Parent forum:" back to the original parent category, I now have the forum in the correct category. But in order to see the forum on the bulletin board, it now seems I have to manually reset the "Copy permissions from:" setting for the forum -- and each and every sub-forum, sub-sub-forum, etc. -- to point to the same category. Yikes!
Is there another, more automated way to do this?