Plutonium

Upnp is enabled. I tried this on both multiplayer and zombies. On the multiplayer I made a private game. When my friend tried to join me there was just a black screen with "Awaiting connection" or something along those lines and then it proceeded to kick him out with the message "Server Connection Timed Out". I then joined a normal server and friend was able to join me fine. In zombies i did the same by making a private match. My friend tried to join me but clicking my name in friends list did nothing. Tried to connect to my ip address and also nothing happened. Then we both tried to connect to a normal server via the server browser and it worked fine. Anyone know what the problem could be?