[TheLastSuperman]

Do you have it set to allow secondary usergroups to interact w/ primary permissions?

*Edit one of the usergroups, then scroll down to find "Usergroup Options" where it says:

Allow Users to have Member Groups
(Sets whether or not permissions from the user's member (secondary) user group memberships are inherited or not)

IF its set to yes, this could be a secondary usergroup permissions issue but that depends on if the secondary usergroup permissions are set to access aid forum or not, if not then of course this was the issue (maybe a user in question was in one usergroup and also assigned a secondary which prevented that from showing somehow).

Permissions can be confusing initially, usually though once people realize usergroup and forum permissions are different yet work together they grasp it fully - just be sure something simple is not making this occur. You may also want to check your plugins, see if you have any plugins that modify css based on usergroup or similar. Edit: Any mods by Valter (aka Cybernetic)?