Can't join my friend on black ops. everyone else i can?

When i try to play black ops with one of my friends it won't let me join his session. he will send me an invite but when i click join it doesn't do anything then about a min later it shows "game session is no longer available" or "can not reach host" then it resets me back to the main menu. Also when i get one of my other friends to host and invite us both it works BUT i can't hear my friend i tried to join before but wouldn't let me. WTF? my NAT type is moderate and it used to be open? idk when it changed but it did all of a sudden. is there anyway for me to fix this? i want to play with my friend and be able to hear him without having to have another friend host.