I am having the same issue, excpet the users that are not inheriting permissions are not members of any admin group. They actually are just random members of different OU's, with no common denominator. The way that I discovered this issue was that one of our helpdesk employees, who is not a member of an admin group, can make changes on most of the user accounts, but some she cannot. When I checked on it, the group she is a member of is not located on the security tab of these users and the permissions are not inheriting, while 95% of the other users in these same OU's are inheriting. Make sense?