My facetime works but when video chatting in ichat it doesn't, how can I fix it?

Everytime I try to video chat with someone on ichat my green light goes on but i don't get a notification from them. and when i send a notification it either doesn't show up on the other persons screen or when we do try to connect it says a error has occured oon the other persons screen. The thing i dont under stand is why i cant Video chat but i can facetime with no preblem on my mac.