We have found that few of the servers which are in monitoring does not have metric data in USM portlet.
Surprisingly few server showing data and i am not able to understand why its happening with few servers.
Its so abnormal that i am not able to identify the similarity in the servers which are having this issue, not able to identify that its happening with same set of servers (OS, origin etc).
So there is no similarity and that is why not able to find out root cause and accordingly solution of this issue.
Usually support finds that this is linked to cloning systems or reusing system name and IP addresses.
below is a Kb article that can help you resolve this issue.
It could also be as simple as just restarting discovery server. I have found that over time discovery loses track of the complete list of hubs it needs to visit and will eventually reach a point where it only revisits a small portion of your total number of hubs.
You'll want to pull the latest version of discovery (that's compatible with your UIM version) off the hotfix page and use that.
A prophylactic restart of discovery weekly or more often will go a long way towards cleaning up discovery's weirdness.
Alternatively, make sure discovery isn't crashing part way through the list of hubs - it might not ever be getting to some of your hubs.