It also may be that the printers IP's are outside of the sweep range too.
Possible but unlikely, only way to be certain is to do a scan, then wake 2 sleeping printers and scan again, if 2 more are discovered you have confirmed your suspicion.
Then you would need to play around with the Energy saver settings.
But the scan discovers devices via SNMP. Check that it hasn't been disabled by an overzealous IT guy.
I have CWW installed and it founds only 12 devices out of 30. There are no network problems. No firewalls or failures.
Is it possible that the reason is that printer is in sleep mode?