Group policy

Has anyone ever had problems with group policy not applying all the time? We have a new Dell Poweredge server that we setup as our Primary domain controller. All of our 150 clients are windows 2000. we set up a group policy that sets the users background to a company themed bitmap. For the first week, the policy worked perfectly. Now, the policy works about 95% of the time. Occasionally, a users pc will not apply the policy. I changed the group policy refresh interval to refresh every 45 minutes. This hasnt helped. Any ideas?