If you tried several control tracks and got the same result, try adjusting the car itself.
1. Loosen screw
2. Hold dump door CLOSED and rotate it to the dump position.
3. Tighten screw almost all the way
4. Insert a flat head screwdriver into the screw slot
5. Rotate the entire assembly back to it's normal position WITHOUT letting the screw turn.
6. Fully tighten screw.
John
If that doesn't work it is probably a lack of power.