HWH Jack Won't Stay Down
Problem: All jacks extend, but one jack retracts once the pump shuts off.
Turned out that the solenoid valve's needle valve end (which threads into the inside end of the solenoid valve body and seals into the inside of the manifold w/its own set of o-rings) had the lock nut come loose and unthreaded till the nose was left in the manifold when the valve was removed. If you don't know what the end of the valve looks like, you wouldn't know to go diving in the manifold to retrieve it.
If the nose end can be retrieved, and the solenoid part is still working, you should be able to re-seat the nose to seal against the needle, tighten down the lock nut, and reuse the original valve. You may want new o-rings (there are 5 total of which 3 are "square-rings" rather than standard round o-rings. If the solenoid isn't operating properly, then you will need a new or rebuilt valve.
Hope this helps somebody w/an otherwise goofy diagnosis.
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project