Are you using heavy duty booster cables and heavy duty (at least 850 CCA) battery to jump it with? If you are using the portable jumper battery packs, I have found them insufficient in cranking amps sometimes to start a vehicle with a completely dead battery. Maybe need to put a battery charger on it for a few hours also.
Alternately, you should be able to raise the bed by opening the valve like you were going to raise it with the engine running and then raise it with a jack or by hand if not too heavy. Opening the dump control valve will allow the oil in the cylinder to cycle back into the reservoir and the bed to come up. You may have to block it up when you get it up as there wont/may not be any oil in the cylinder depending on the system configuration.