All of my friends are having this problem too. All of my ports are forwarded and my NAT is open as is everybody else's. :(
Oh that is a common issue. Do is it say you play list is older than the host? Tell everyone in your lobby to restart the game and make a new lobby. That usually works, I've had this issue occasionally in every Cod since 4.
I am having a similar issue. I have one person that I can't invite and they can't invite me. I constantly get a message: Notice
Game session no longer available.