So the switches are all managed in campus and so is the Nexus 7K?
And you have verfied for some servers the the default gateway is in fact and interface of the Nexus?
You can start by checking if the Nexus has the devices you wish to see in its arp cache.
I guess "sh arp" should work on NX-OS. LMS should query the arp tables using SNMP and see the same results.
Cheers,
Michel