Can't find a matchmaking game.

I cannot find a match making game. It says it is looking for other players and then is says no players can be found. I dont have any issues with my network and I contacted xbox live support and did cleared the alternate mac address and did a hard reset and nothing helped. They could not find the problem so they told me to come here. Custom games work fine and I can do everything else online except get into a match making game. please help