Multiplayer joining issues

[664a6ef2e6e60Fironkkuify]

This has probably been brought up before but here it goes anyway.

So there's three of us getting into a game.  Two of us can't host at all for reasons I can't explain, yet the third person can host just fine and we can all connect to that person.

Later on when that third person had to leave, we tried connecting again and still couldn't join eachother's games.

We already checked to make sure that our firewalls weren't being overzealous.  We're also using Tunngle.

Does anyone know why some people can host just fine while others can't host whatsoever?  I used to be able to host just fine but I can't anymore.