Computer groups with Security Filtering

I want one of my group policy to only be applied to a certain group of machines in an OU.  I created a group for these machines and added these machines to the group.  I then linked the GPO to the OU that has all of our servers in it put the group under Security Filtering.  I did a GPUpdate /Force on one of the machines in the group and it is still not getting applied.  If I add the machine in under Security Filtering and do a GPUpdate it does get applied.  What am I doing wrong or can I not do this?