Snow Leopard vs Lion connection issues?

I am running two MBPs (mid 2010 and early 2011) connected wirelessly.

I am not sure whether this has to do something with Lion and Snow Leopard being installed on either one of them but my problem is that even though they can see each other at times, the connection doesn't work. Firewall works, exceptions have been added but some things don't work. When one Mac establishes a Warcraft III game, the other one can see it but cannot connect. When I played Trackmania Nations Forever port for Mac, the exact same thing happened. It's like they see each other but can't really connect. I can place things in SL's dropbox, but that I can see from time to time. Ping works both ways.

Does anyone have a clue what's happening here because I am out of ideas?

So it wasn't a Lion x SL issue since both MBPs run on Lion now. Airdrop works, connecting to the other Mac's public folder however doesn't. At least not all the time.