The most common problem with devices disappearing from groups deals with ACS integration. If you are integrated with ACS, make sure that all of the devices in the topology group in question are known to that ACS server, and that no one is moving the devices out of ACS or into inaccessible NDGs within ACS.
If that is not a factor, then it may be time to do OGS debugging within LMS. Unfortunately, this is not as easy a processes as with other debugging. It would be best to open a TAC Service Request if it comes to that.
