Call of Duty: World at War

I've been having trouble connecting with one of my friends. I can connect and play with other friends just fine, but just not that one. We start in a empty lobby and try to invite one another and we keep getting different error messages. I can invited another one of my friends and they come through just fine. We have deleted each other and resent friend request but that didn't help. Have any of you had this problem (and or) know how to fix it?