User Rights assignment applies to individual users but not Groups

Hi

I have a problem with user rights assignment GPO that I cant figure out.

I am Running Server 2012 R2 with same domain functional level.

I will use "Shut down the system" as an example.

I have defined the policy and added on administrators and domain\domain admins on to the policy, but this does not allow any of the users part of the 2 groups or both of the groups to shut down\reboot the test machine, only to log out, but as soon as I add the individual domain\username on to the policy the said user is able to shutdown the test box.

From what I picked up, any of the user rights assignment policies I cant use any security groups but only individual users, am I missing something?

Thanks