I have an SR101 router, which after help resolving issues with a while ago has been just fine forwarding ports necessary for a long time now.
But recently i was getting NAT type of moderate in Black Ops II, so i investigated.
Turns out UDP port 3074 will not open...
I tried UDP/TCP, and just UDP re-additions of the custom service to the firewall rules, to no avail.

I'm a little stumped as to whats causing it (definitely not anti-virus before anyone suggests it).

Any light on this would be greatly appreciated