1. what does your /etc/nsswitch look like?
2. does the user in question exist in the /etc/passwd file on both the client and server?
suggestions without knowing the above:
in /etc/nsswitch, you should have something like:
passwd: nis files
group: nis files
shadow: nis files
and the user should probably not exist in the client /etc/passwd.
also, be sure you "make -C /var/yp" when you add new users on the server.
Also note that after you get this working, the user will probably not be able to log into the client machine as their home dir will no longer belong to them until you straighten out the owner (it will be mapped to the old UID).