I found a fix for my problem:

  • I found my answer by reading through the Linux Pam Administrator Guide
    I edited /etc/pam.d/su and uncommented the following line:
    Code:
    auth       required   pam_wheel.so


This line sets the configuration so that a user must be a member of the "root" group in order to issue the "su" command. I'm not sure why I had to do this as with the line commented it should allow all users to issue the "su" command. Anyhow, it works now.