You may received the following error when try to logging in su,

#  su -
su: cannot set groups: Operation not permitted

have to check the su file permission in /bin directory  : /bin/su
# chown root:root /bin/su

# chmod u+s /bin/su

Once updated that file permission should be as like as below,
# ls -l /bin/su

-rwsr-xr-x 1 root root 34904 Jun 25 14:25 /bin/su*

Now, you can logging into su.