Ohhhh... I'm a moron.
So if they're carrying enough to fill up the garbage dump, they won't carry any more, unlike food, where ant will pick it up and just drop it if the food pile is full. So minims work differently from normal ants. There was space in my dumps, so I assumed the ants weren't doing anything for no reason, when in reality the cap had already been hit because the minims were holding the garbage needed.