some IP addresses return a hostname while others do not despite nslookup returning a hostname for each

Disclaimer--I am still learning IPAM.

Why would we not see a hostname for some IPs in "Manage Subnets & IP Addresses" page when an nslookup from the SolarWinds server returns this information?

For an IP address of something SNMP-monitored in NPM, why would we not see relevant info about that device in IPM inherited from NPM?  Hostname, for example.