A similar thing happened to me, while I was at the dump. Trailer went up, but would not go down. I had to crack a fitting to get it to go down.
I checked everything, and was about to replace the soleniod valve that returns fluid to the reservoir, when I decided to check the wiring on the plug that attaches the controller to the dump trailer. Sure enough, it was shorted out.
Now exactly the same thing can happen with the "raise" function as the "lower" function. Check out all of the wiring on the control unit, especially on the plug. Mine was good quality components, but a very poor assembly job. This is a lot cheaper, and safer, than working on the motor or solenoid, and might just solve the problem.