Network - Receiving NAT punchthrough failed

Hello,

I have a semi working game that I made into a build and sent it to my windows buddy. He sets up a game to host on his windows. I try and connect (His game pops up in the lobby), but when I try and join, I get a 'Receiving NAT punchthrough attempt from target ... failed'.

I'm not 100% why or what this means, but my game works internally from Mac to Mac, and externally from Mac to Mac. Not sure if its OS related, but I doubt it.

Anyway, does anyone know the reason for this? Bearing in mind my lobby can find his game list.

Thanks

PS, it doesn't work the the other way round either, so I Host, and he tries to join - no go.