When you use Custom AIs in games where the starting condition are imbalanced, the Custom AIs do not attack or build because as they view the condition as not being able to win. You have to use Insane AI for them to attack you and build, when using the AIs in Melee games, the computers sees a clear disadvantage to them so they do not even bother to built/expand, this does not happen all the time but it does happen. The best AI to use is Insane AI. The campaign AIs only build the mimium requirements to advance the tech tree and collect resources very slowly (4 drones max, only one hatchery, while Insane AIs build 2, Insane human AIs built more than 3 unit producing buildings each, like 3 factories, stargates, etc.)