Su not working correcttly
 

I'm currently having a problem with su on a webserver i have.
I ssh in as an ordinary user eg shagman
i can then successfully su to root.
However i have a problem trying to su to another normal user (even when root).
No error is displayed, It simply asks me for the password, i enter it and it leaves me as the user i was orginally logged in as:
eg:
shagman@server:su testuser
password:
shagman@server:

Anyone have any idea what could be causing this, i am using redhat 9

hmmmzz.... all i can think off is:
is your /etc/passwd healthy (data and/or rights(chmod 644 /etc/passwd && chown root.root /etc/passwd)
and maybe also your /etc/shadow (if used)

cheers