Remote Desktop Enabled via GPO but only working on some workstatio

I have added a new GPO to my workstation OU but after confirming the GPO has
been applied to the clients only some of them allow a connection when others
do not. All workstations are XP Pro. Also, is there a way to enable Remote
Desktop via GPO and allow only Domain Admins access?

Any info is greatly appreciated.

Max Rawson