some servers are not present in cm_nimbus_robot table because of which their QOS metrices are not visible in UMP.
Most of the views in USM do not use the cm_nimbus_robot table but rather the cm_computer_System table.
I would check the make sure you have probe_discovery queue from all of your hubs with a path back to the primary.
I would also increase the discovery_Server loglevel to 3 and check the logs for communication errors from the discovery_server to the problem robots and resolve that issue if it exists.
Last I would do a discovery reset for the individual robots if the exist in cm_computer_system but not in the cm_nimbus_robot and allow them to re-populate
of you are on UIM 8.51 you can do a discovery reset by finding the system in USM and using the option to delete
or follow the below KB article:
Are you facing the issue for most of the servers ?