Spammers are the real reason of this problem. To discourage spam-email address hunters sometimes some registrars stops showing who-is results. Even sometimes after showing two to five who-is results same registrar website shows error message stating 'very high traffic generating from your IP address'.
It happened many times with me when enom did not show any result but 'register.com' did . When it happens, try some other registrar, you'll get the info.
Regards