I have a 2000 Discovery... And have had some issues with the jacks along with lots of other systems
My Discovery has provided lots of enjoyment but not without quite a bit of tinkering. The powergear jacks used on my Discovery are not unique to the coach and seem to be pretty commonly used.
The key must be on and the tranny should be in neutral with the parking brake on. Doesn't matter if the engine is actually running or not. Does the control panel light up when you push the on button? If not, check for a fuse or wiring issue.
If it lights up and you can hear the motor run when pressing any of the down buttons, then chances are the pump has sheared the gears within it. Mine did this and I had to purchase a new pump. I removed the entire assembly from the coach and cleaned and installed the new pump. I got this off ebay (Power Gear 800301 Pump Only Service Kit for Hydraulic Leveling). This is one of the more expensive fixes that I had to do to my coach (so far
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If you can't hear the pump running then there could be a wiring issue between the control panel and the pump. You will need to check the voltage back at the pump to see if it has voltage.
Hope this helps, and good luck!