The default triggers on a map are just like in a melee game. If you control no buildings, you lose, if your enemy controls no buildings you win.
I'm guessing that you still have those triggers in the map (you want the game to play like a melee, right?) If that's the case, you need an enemy with buildings.
If it's human vs. human the players may be allied with eachother at the start, so you all just win instantly. You need to make them start as enemies.
Go under the Force settings and uncheck the "Allied", "Allied Victory", and "Shared Vision" boxes for whatever force the humans are in: