Example scenario (no metrics displayed in USM for one or more robots):
In one particular customer case, where there was no tunnel between the primary and remote hubs, the discovery_server relies on the nametoip callback, and gets the address back in the result (e.g., 10.24.x.x ) and then attempts to connect directly to that IP, port 48000.
In the discovery_server log you must search for errors regarding the given robot hostname and/or other hosts for which no metrics are being displayed in USM. As an example this robot below shows the type of error you would expect to see in the discovery_server log indicating that it was trying to communicate directly with the 10.24.x.xxx robot to fetch the niscache elements but it failed.
Error showing failure to fetch the nis_cache elements on the given robot:
15 Jan 2016 17:37:02,322 [robotWorker-2] WARN com.nimsoft.discovery.server.nimbus.scan.NisCacheUpdater - fetch nis cache failed on pass=0 with 0 total elems received for /HIXXX/XX_Secondary_Hub_Servers/<hostname> : (80) Session error, Unable to open a client session for 10.24.xx.xxx:48000: Connection timed out: connect
The discovery_server log should be set to loglevel 5 with a large logsize, e.g., 20000 and check the actual log on the file system to be sure.