can't "su" to user account
I have a user account on my RHEL 4 server (yes 4, it's not a typo). His password expired so I changed it for him. Since I did that, he is no longer able to ssh to the server.
When I am on the server as root, I tried to run "su - useraccount". It immediately errors out with "incorrect password".
I verified that his account was not locked, I tried resetting the password again, and can see the last password change on the account is today. The shadow and passwd entry for this user are fine, and I don't see anything in pam.d/su file that would prevent this.
Does anybody know why I can't su to a user account from root?