nis client machine can't log in using nis username
 

hi
i configured nis server/client for 6 machines on a same network. while configuring there was no error on server side as well as on client side .on client machine ,using ypwhich command i can see ,server's hostname. using ypcat i can see nis user database . it is same on all client machine but not even a single machine can log in using nis username and password..it is coming with an error message "invalid username or passwod please try again." what should be my next step to solve this problem?
waiting for + reply.