problems with baggage system

Everything in my systems seems to work fine, but my baggage carts don't pick up the last batch of bags at the depot before the plane leaves. For example, flight 551 is at the gate with 34 out of 40 bags loaded. The other 6 are at the depot assigned to that gate, but the carts go back to the hangar and don't pick them up, and the plane leaves. What am I doing wrong?