Almost sounds like a morale issue to me—I’ve found sometimes this happens if the folks on the floor feel like management is against them, and so they start to stop caring. I would go less for stick (which would decrease morale further*) and more for carrot, especially if things are more “team” builds—e.g., a group of people take a product from beginning to ship. Offer a bonus (could even be relatively small—maybe 20 pounds) for each order shipped with no errors. If it’s the team doing it, they’ll start to watch out for each other more. (* “the beatings will continue until morale improves”)