Rick is absolutely correct.
A UDS works based on induced draft. Hot air rising from the burning charcoal exits the exhaust at the top of the drum and the draft "pulls" air in through the intakes. That's why when you take the lid off, the temp shoots up because you get a whole lot of draft then.
If the racks are too full, you don't get decent draft out of the exhaust and then you have no induction to pull air in through the intakes.