This happens A LOT. I think they are just gonna tell you that it is YOUR NAT type that is the problem. I call shenanigans on that. Honestly this is probably a problem with their matchmaking servers. I mean 1 million + people online and it can't put you in a game that all 6-9 of your party members can all be in? And it isn't just full parties that I have this problem with. It happens when I have 2,3 and 4 players almost as much or MORE than if I have a full party, from my experience.
Also have you guys had the problem where host migration doesn't work half the time? Like the one mid game when the host quits out, where it is supposed to stop you from losing all your stats and going back to the lobby. In all of my games I would say AT LEAST 75% of the time the host migration tries for 10 seconds and then we get kicked into the lobby...Real great work fixing that problem IW (/sarcasm).
I don't know, I will support MW2 cause I enjoy playing the game, but I swear there are more problems with the "features" they have added to the game than, it seems, they care to admit. I really just wish they would say; "Look, we know there are problems a, b and c. We are working on a patch to fix them and it should be available soon." That way we might actually have some hope of the problem being solved in the near future.