If you have not retracted the lower leg first by pulling the pin and lifting it (once it is on the truck of course). Do that.
Also assuming the motor runs up and down, but stops at a certain point (fuse not blown).
Most landing gear have a Zerk fitting on the front top of each leg. You can try lubricating it (see photo).
If that is not your problem, you could have a broken tooth (teeth) in the screw jack (or top pinion gear) inside the leg itself. You will need a shop to replace the landing gear leg.
Lou, Laura, & Freya the wonder dog
2008 GMC Sierra 3000HD Crewcab SB Allison Duramax
2019 Flagstaff 8529FLS - Pullrite 3300
HAM CALLSIGN - KC3FFW