Login from X-terminal

I have a Solaris 8 system acting as a NIS slave. My /etc/nsswitch.conf file specifies

passwd: files nis

and in general users can login successfully. However, I have a few users who use X-terminals booted off the Solaris box and these people are unable to login unless they have entries in the /etc/passwd and /etc/shadow files.

It looks like for these guys the login process is not using NIS.

Any insight into this problem would be most welcome as this is driving me nuts!

Thanks