on your OCS front-end server, open a command prompt and navigate to the location of the abserver.exe file. Execute an abserver.exe -syncnow (as I recall) and then monitor the event logs for OCS activity. Server 2003 will throw them in the application event log, server 2008 and R2 break them out into their own event log, so make sure you are looking in the right place. You should see information events for successful syncs, and errors/warnings if there are problems. If you see *neither* then you aren't looking in the right place.
This sounds like the server not updating the OAB on the server.