problem with X11Forwarding

[Mahmood Naderan <nt_mahmood@xxxxxxxxx>]

Hi,
I have enabled X11Forwarding in sshd_config

mahmood@server:~$ cat /etc/ssh/sshd_config | grep "X11*"
X11Forwarding yes
X11DisplayOffset 10


However when I connect through "ssh -X server_ip" and run "xclock" I get this error:

 mahmood@server:~$ xclock
Error: Can't open display:


Any way to fix that?
// Naderan *Mahmood;