Another possibility. The locking pins are like the sharp end of a pic-ax (semi-sharp to blunt). You can see them in the Figure 12 also inside the legs of the slider.
Undo the hitch pins at the bed rails. Roll the hitch forward or backwards to expose access to the bottom of the side of both legs. Spray heavily inside this area with WD40. This will lubricate the "leaf springs" which the locking pins ride and assure when you move the handle from Tow to maneuver that the pins are properly and fully releasing.
The arm will move much more smoothly.