Yeah, this is a slightly annoying quirk of MyBB . Unlike ZetaBoards, it doesn't assign you your new group automatically: instead, there's a process that runs every 15 minutes to promote people based on their post count. So, when you hit a post count requirement for something, it might take up to 15 minutes for you to get the associated rank and/or privileges.
In any case, I'm glad it's worked now . And, since there's not really anything more to add, I'll close the thread.