is there any event log information on the workstation showing the gpo failed to apply for whatever reason? why not apply the gpo at the computer OU level rather than at the root of the domain.
Also, run rsop.msc to determine what gpos are being applied to the workstation and see if your's is in there. I had a gpo setup for users to apply IE proxy settings and they weren't getting applied. Determined it was caused by the order my gpos were being applied. changed the order and it worked.