Problem Video Chatting on iChat

I am having trouble video chatting on ichat. On skype it works fine and when I make videos it works fine. I just get an error when I try to video chat through ichat.... can anyone help me out????? I really need this to be fixed. I took it to the apple store and my ichat worked at the apple store --- so any ideas of what it can be???? or what should i do? thanks!