Exactly which version of Campus Manager are you using? For Campus to be able to acquire the IP address for a given host, a router on the same subnet as that host must be properly Data Collected, and show as green with a proper icon on the topology map. This router must also have the ARP entry for this host (enabling Ping Sweeeps in Campus Manager is a good way to ensure this).
Once the IP address has been acquired, then UT will attempt to resolve that IP to a hostname using the system resolver (assuming you have enabled reverse DNS under Campus Manager > Admin > User Tracking Admin > Acquisition). Once you have an IP acquired, you can use the command:
NMSROOT/bin/perl NMSROOT/bin/resolver.pl
To test to see that resolves to the desired hostname.
To troubleshoot this process you should enable vmspadmin debugging under Campus Manager > Admin > Campus Data Collection > Debugging Options, then run another User Tracking acquisition. When it completes, search for one MAC address that is missing its IP through the ut.log. If you don't see any errors, search for IPs of routers that should have its ARP entry, and make sure they are being properly polled.
