We can’t get the back form roller on the SP-15 to lower on the gear side only–it can’t go lower than a narrow sliver (less than 1/16″) on the roller setting gauge, no matter how much you turn the knob.
We can adjust this back roller’s height on the operator side, and the front roller adjusts fine as well–its just the side that has the gear.
The most likely cause is that the diameter of the form rollers are significantly undersided. It should be 2.5″.
The gear may be bottoming out in the rack, while the rubber is greater than 0.918″ away from the bed.