Check level of oil in pump reservuoir. Check to see if the cylinder shaft is bent, thus binding in the packing nut, not allowing the jack to come down. You might loosen the nut on the hydraulic hose, on the cylinder, activate the pump, see if any hydraulic oil squirts out. Likley it is transmission oil, it will be red, easy to spot. DO NOT put your hand near the fitting when doing this. IF there is oil, it will be high pressure, over 1000 psi. it can directly inject through your skin, resulting in loss of limb, and or life.
Was the jack ever extended on a very un-level surface? This could cause the hydraulic ram to bend and therefore bind...especially if it was extended a great length.
If you have oil at the fitting, under pressure, it will most likley be an internal cylinder failure. Most of those are not re-buidable, throw away, fetch a replacement!
Let us know...
Back to a 5'er after 4 yrs fulltime.
Pulling with a '14 2500 ram, 6.4 Hemi.
Montana High Country 31RE