question about armies in editor?

I have this scenario and it works nice and well, but about 12-15mins into the game the comp units that i army deploy just end up standing around, only walking sometimes, and are very unresponsive when attacked. Why is that? Is there a unit limit that breaks the ai or an army deploy limit or something? it only happens i assume half way into the game when there are tons of units spawned. (its a survival map and i loop 16 different army deploys every minute or so)