Cannon, you could be right. I bought the camper used with the jacks on it. Previous owner did indeed back into something, damaging the mounting plate on the back corner of the camper -- fixing that corner is one of the jobs on my list.
On the other hand, 3 of the jacks work fine, and the jack in question was on the front right side, which shows no sign of damage. And I can't detect any deflection in the stuck jack. But yes, with tolerances so tight, that could certainly be the problem. And previous owner might have swapped it around.
Glad to hear JimK got 18 years out of his. That would do me.