Imran -
Are the missing devices discovered through scopes set up in a Discovery Wizard of the UMP USM portal? Or are these devices robots or discovered because they are being remotely monitored by a probe on a robot in your UIM domain? If they are discovered by the former process (scopes set up in a Discovery Wizard of the UMP USM portal) and you do not have this set up to automatically rediscover the devices in the configured scopes, then you must kick off a rediscovery from the Discovery Wizard.
If the devices are still not rediscovered after kicking off a discovery from the Discovery Wizard, or if the devices are robots or are remotely monitored by a probe installed on a robot, then it will be necessary to collect discovery logs. If the missing devices are discovered through scopes set up in a Discovery Wizard, you need to change the loglevel to 5 (logsize to 80000) of the discovery_agent responsible for discovering these devices and kick off a new discovery. The discovery_agent.log files should determine why the devices are not being rediscovered.
If the missing devices are robots or remotely monitored by probes installed on a robot, then you need to change the loglevel to 5 of the discovery_server, restart the probe then review all of the discovery_server log files to determine why the devices are not being rediscovered.
If you cannot determine this yourself from the appropriate set of log files, then I would highly recommend that you open a support case to get assistance with reviewing these files to determine why you cannot discover these devices.
Hope this helps.