Recently I’ve been in an awkward situation with ssh and X11 forwarding, where ssh using the classic -Y (or -X) switch wasn’t working (DISPLAY was not set):
[andrea@duncan ~]$ ssh -Y root@thufir
Last login: Sat Apr 12 23:30:09 2008 from ***
[root@thufir ~]# echo $DISPLAY
If you are experiencing the same and your sshd_config looks fine (which means TCP and X11 forwarding is already enabled) then the answer is missing xauth!
yum install xauth will do the trick