It's hard to tell why, as Windows supports several ways of resolving IP addresses back to domain names.
You can try different name resolution methods under
Options - Additional - Resolve host names.
If that doesn't help, try opening a command prompt and typing
tracert x.x.x.x
where x.x.x.x is the IP address whose host name you want.
Normally it would display the host name in the first line of output, like this:
C:\Users\Name>tracert 192.168.0.22
Tracing route to NAS [192.168.0.22]
over a maximum of 30 hops:
1 2 ms 1 ms 1 ms
...
If it doesn't, I guess something may be wrong with your network settings or firewall configuration.