Additional Usergroup don't see forums

[CarpHunter]

I've searched for this but couldn't find anything that looked like this problem.

Situation:
I have a clan forum where 4 differen't divisions (sub clans so to say) are active.
All divisions have seperate forum places for posting there stuff.
Some users are member of more than 1 division and need forum access to those forum parts.

Little illustration:

X = usergroup X
Y = usergroup Y
Z = usergroup Z
1X = user 1 primairy member of usergroup X
2Y = user 2 primairy member of usergroup Y
3XZ = user 3 primairy member of usergroup Z and additional member of usergroup X

Forum 1 || X
Forum 2 || Y
Forum 3 || Z

Now my problem:
I thought I could easy do this by adding them to an Additional Usergroup that has forum permissions.
Problem now is that they only see the forums from the Primairy group and not from the additional. (user 3 see's forum 3 but doesn't see forum 1)

Is there something wrong with the way I've set it up?