For some reason, we reinstall VMM (3.1.1-416) with existing UUID and rediscover all VMSs. The process went smooth and the whole system looks clean without dead objects any more. The guest VMs are all working fine. However, the new added VMS in unassigned servers are not displayed in the "Available Servers" when we want to add into existing clustered server pool, but it is shown when we add into unclustered server pool. Anyone has clue or possible way to debug what's wrong.