Did you check for fuses in the control box? Or out near the pump/tank assembly?
If those are OK, what you could try is to measure the resistance of the solenoids. If they all measure fine, then swap wires from a working jack to the non-working jack. If the moves to a different jack, then the control box is probably bad. If it stays on the same jack, might have a stuck check valve in the hydraulic assembly. But first, it wouldn't hurt to check that the contacts in the connectors are all tight. Then squirt some contact cleaner on them.