It is normal for the arm on the PocketRIG 2 to move inwards about 1.25 cm when first tightening the screw and pressing it on your shoulder, so once it locks in place, it should not move inward anymore.
When you are finished with the PocketRIG 2 and need to fold the arm again, you should first extend the arms out by pulling them in opposite directions, thus making it possible to loosen the screw and fold the arm.