Help! One of my jacks won't retract or extend!

Unless you know how to repair it, want to tear it apart and learn the quickest and easiest way I surmise is buy a new jack, jack up that front corner and replace it.
Or jack up 2 corners and swap one from the rear.
That’s a bit more risky as I’ve not loaded ours with only 3 jacks. I just know it will float a rear jack regularly and seems to support well that way.