Ichat Av


I am trying to get an audio video conference with my brother going. We both have imacs and isight and ichat av. We can use instant message but not video conference. I looked at the firewall on my computer and look care of the ones mac.com recommended. I called optimum online and ask them to check the line and my modem (Webstar DPC2100) and they said their were no firewall problems. Please advise?