There may be several reasons:
- you are creating a game on a map where one of the players does not have a starting town;
- firewalls are active on both or one of the players at the time of creation;
- the Hamachi adapter settings are not prioritized in network connections (not sure if I'm using the correct terminology). To do this in Windows XP: select "Advanced" in the network connections folder menu, then "advanced settings," and use the green arrow to move the Hamachi network to the top of the list of network connections; in Windows Vista and Windows 7, go to the network and sharing center, change adapter settings, press ALT, and in the menu that appears, repeat the steps for Windows XP.