I’ve seen this one on the VMTN forum a couple of times. When the Health Status isn’t showing you could do the following to fix it:
Restart VirtualCenter service on the VC Server
Restart mgmt-vmware service on the hosts that are affected (service mgmt-vmware restart)
Restart vmware-vpxa on the hosts that are affected (service vmware-vpxa [...]





vSphere 4.0 Quick Start Guide