We have a 2001 Damon Intruder with the HWH leveling system, joystick control next to the driver's seat.  There are two arms that I swing away from each other and then there's a center joystick that I use to extend the jacks.  When I bring the two opposing back towards the middle that releases the hydraulic fluid so the jacks can retract

Three of four behave, driver's side rear does not.  I have to wedge a prybar underneath and work the jack back up.  Pry it up a few inches, jam some wood underneath, pry it up a few more, repeat until the jack is fully retracted

I have disassembled the joystick control station and the hydraulic valves are working but for some reason the one jack is stubborn.

I called HWH and par for course their next available phone tech is four week out.

Short of replacing the jack is there anything else I can check? The piston is clean and shiny

Spring retract? Or power?

Spring: lack of power to retract valve, bad valve, binding cylinder.

Power retract: lack of power to retract pump or valve, bad pump or valve, again, binding cylinder.

I once did a road call to raise the jacks on a motor home so they could leave.
I'm sure it was an interlock  problem but I could not find the problem in the time allotted. I jumped the motors at each jack, with instructions to not use them again until they were fixed.

They were happy to be back on the road.

Spring retract.  I have to wedge a prybar under the foot, pry it up an inch or so, put a 2x4 down, wedge the pry bar back under the foot using the 2x4 as a fulcrum, pry it up, add another 2x4, etc.

I keep adding 2x4's until I have enough height to pry the foot back home.  Comes down just fine

The fact that you can pry it up by hand seems to indicate that it's releasing hydraulic pressure on the cylinder properly.

So - either binding cylinder, or weak retraction spring.

If you were feeling ambitious, you could swap it with the passenger's rear side to confirm that the problem follows the jack.

This for sure..  

Another thing to do is inspect the jack shaft for grime and goo and clean the crap off of it and lube it.

I clean my HWH shafts with WD 40 when they get sticky

Also do your springs have the tapered ends?  Might be time for new springs

This is my thought, new spring time! Go ahead and do them all while you’re at it.