Mw2 on the xbox 360 cannot find any games

Hello,
First off, my NAT type is open and i have the xbox 360 plugged into a router. When I click on find match and any play list, it will keep on searching for games but I can never join one. Call of Duty 4 works so does every other game for going online but not modern warfare 2... please tell me how I can fix this problem.