nis auth problems

[FreeBSD at TinyCreatures]

Hello.

I recently set up a NIS server on a box running FreeBSD 5.2. Everything
went well. The clients are Linux machines, tried to configure one, it
connects to the NIS server, all yp commands return successfull queries,
made the necessary changes in nsswithch.conf as well, however, when i'm
trying to log on with a user that exists only on the NIS server i get
"Login incorrect" messages all the time.
What could be the problem.
Thank you.