Lostplanet22 on 27 October 2010
People are still playing L4D online on X360 so the userbase can not be the problem for LFD2.
Your location/NAT may aswell not be an problem if you can play other games online...
Edit: Checked gamefaqs for you:
http://www.gamefaqs.com/boards/960511-left-4-dead-2/56755691
You are not online with this problem
