I'm not entirely sure why but no matter how many crew i cram into an area near a ton of factories, they all just swarm a single one, and if i tell them to only use a factory other than their default they just refuse to do anything!
This problem is particularly annoying with Warhammer40K but it does happen with every other ammo producer in the game. I can more or less bypass that for vanilla ammo types with Automation++, but it's still a pain to deal with.