There are many possible reasons for a user being unable to log in:
User forgot password. To set up a new password for a user who has forgotten the previous one, run passwd for that user on another machine (naturally, you have to be the NIS+ administrator to do this).
Mistyping password. Make sure the user knows the correct password and understands that passwords are case-sensitive and that the letter "o" is not interchangeable with the numeral "0," nor is the letter "l" the same as the numeral "1."
"Login incorrect" type message. For causes other than simply mistyping the password, see "Login Incorrect Message".
The user's password privileges have expired (see "Password Privilege Expiration").
An inactivity maximum has been set for this user, and the user has passed it (see "Specifying Maximum Number of Inactive Days").
passwd: files nis
passwd: files nisplus
passwd: compat passwd_compat: nisplus
Any other configuration will prevent a user from logging in.
(See "nsswitch.conf File Requirements" for further details.)