It should just be a combination of Moderators having the permission to see the forum, and the moderator actually being assigned to be a moderator of the forum.
Just to clarify, Moderators won't get the Moderator permissions for a forum unless they are actually assigned as the moderator of the forum.
If you've done that, which it sounds like you have, then I'll probably need a ticket/pm with some details so I can look into it further.
