Xbox 360 connections and help?

hello whenever i want to play any game with my friends online no matter what it sais can not join. i have the gold membership and i have an xbox 360 wireless adapter. i simply go online my friend sends me an invite and i accept. it trys to join and then it sais can NOT join. my NAT is open and everything is setup. CAN SOME ONE PLEASE HELP!!!!!!!!!!!!! PS: i have the xbox 360 elite. GIVE ME INSTRUSTIONS!!!!!!!!