The first things I would look at would be your policy structure. Where is the policy applied, at the domain level or by OU? Is there a common factor among the computers not receiving policy such as, are they all part of the same group or OU? If so, check your policy links. Are you blocking policy inheritance at any level?
If your satisfied with your initial config, use the GP Results Wizard (GP Management mmc => GP Results)and run it against the computers that are not getting the policy. This tool will confirm or deny that policy is being applied to a given computer. It is possible some of your computers are simply not receiving GP properly or at all. If that’s the case, don’t rule out checking your communications path as it could be related to a faulty port or ports on a switch.