[SSSD-users] sssd not authentication with user in random groups

Good afternoon, I've been playing around with sssd for a while not and it's been great but I've just run into a really weird problem. If I have a user specified in the 'simple_allow_users' configuration directive it works absolutely fine BUT I've got (at least) 2 groups that, for some reason, if the user is a member of these groups the account can't authenticate on my boxes. The groups that I'm having problems with are nothing to do with any simple_allow_groups - they're just normal AD security groups... Can someone please point me in the right direction on this one or let me know how I can best find out why these groups are affecting sssd? I've been trawling logs but can't seem to find anything obvious. Thanks, Allan