Plutonium

originally we thought the servers were down. we can see each other are on and are in a party but when we try to join off a invite or if our party is open to join the game doesn't allow us to join. we've done this between 3 people it still doesn't allow us to join. we use black ops 1 and 2 zombies and multiplier the issue is still persistent if someone could help us out it would be appreciated.