Could be a couple of things. Stern has service bulletins on this sort of thing and other JP problems. First I would go into the diagnostics and check the T-Rex operation. If all good I would go straight for the T-Rex himself. I had the same problem on mine. It went left and right and down, picked up the ball and right near the top of going up it would drop the ball. Took it apart, not as had as it might look like from the diagrams in the manual, and what do you know the wire to the solinoid for the jaw was breaking. Looks like it had broken once or twice with all the electrical tape around the wires. The T-Rex bends in the same place all the time causing the wires to fatigue and break. This means that when it reaches a certain place the wires come apart and you lose power to the jaw coil. A quick solder and some moving of the wires and it was back in business.
Not saying this is the only thing to cause the T-Rex not to function, but that would be my guess. You might also look at the switches for the up and down movement, might be that it is sensing that it is at the top before it really is.
Read the service bullitens and then take a look at that T-Rex.
Good luck