Some computers stuck at "Pending first discovery"

Hi,

cluster with 4 hosts, running vcenter 5.1, esx 5.1, and IN 2.0.

the Navagiator has enumerated all the hosts in the cluster with the exception of three hosts (all are MS SQL vm's as well)

each of the three hosts is sitting at 'pending first discovery'

they have stayed this way for a couple weeks, and through reboots.

the vmware tools are installed and communicating with vCenter normally.

where do i start to look for logs or other indications of what might be going wrong here?

Thanks

Jared