Active Directory Syncronization - users & groups

Hello,
I'm working on a WSM 4.0 test environment composed by an Headquarter and a Linked Site. I noticed a problem, that occoured with version 3.6 too.
When I add a new Active Directory user to a group that is imported in WSM, or when I change the membership of a user from one group to another one, both imported in WSM, user group membership is not updated in WSM. Even if I delete and re-import the group in WSM, users in WSM does not match those in Active Directory: the only "solution" I found is to delete the group in Active Directory, create a new one with a different name, update the membership and import it again in WSM.
AD groups are defined as Global Security groups. I tried setting the primary group on each member user as the group imported in WSM, but nothing has changed. I tried reinstall WSM without changing the default 600 seconds syncronization polling frequency (changing that value will cause WSM not to sync with AD, and this is a known bug), but nothing as changed.

Has anyone reported the same problem, or can anyone help troubleshooting AD sync issues? Any suggestion will be much appreciated.

Regards