CoD WAW unable to join Co-Op games

[vail3r]

I have been trying to play co-op online mwith a friend, it was working yesterday but now i am unable to join his game, he is also unable to join my game if i create one. The same thing happened the other day too. The game is on steam too.

Any ideas?

 

[BigRazer93]

port forward mate on your router, had to do this with mine.