How close are the creeps to your base? What type of map are you making?
You might want to check the creeps' guard distance. Select them all, press Enter, and you'll find, on the bottom right of the window that just spawned, two options regarding the Guard Distance, which will be, in the majority of the cases: Normal (600) and Short (200). Set them to the latest. I'm not sure if this affects buildings, I don't think so, but try anyway.
Are you sure you've explored the gameplay constants? I'm sure there's an option to relate creeps' aggressiveness to building.