Just to expand on Rufus's comments
Are you referring to the canned report 'Server Availability' ? That report is based on the agentless metric for ICMP monitoring. If you look at the report in XML view you can see it references the
GUID = 695A6729-C8B3-4722-8542-0148073DD302
and in instance of STATUS
I would check to make sure those servers are being picked up by agentless monitoring.
My $0.02