Are you trying to connect two computers over a LAN or is one person going to the internet and starting a server and the second person trying to join?

If the latter, I know what it is: You cannot have information flowing trough the same ports two directions, so it is clogging. This happens very much in Age of Empires and other major games. Two people cannot play the same game on the same IP.

Solution 1: Get another IP. More expensive, but the fastest fix. Perhaps the only fix.

Solution 2: Play on the same computer. Two players with one feed. Not too good, but can work.

Solution 3: Play two different rooms.