GPO Trouble

I am running a Windows 2003 Server with GPMC installed on it. I am having trouble getting a GPO to apply to a Security group in an OU. I have added the group to the security filter ing section located in the scope tab of the GPO detail screen. The GPO works fine for users in the OU but I have tried everything, with no luck, to make it apply to the Security group. Any ideas out there?