I think it is not because the stop is worn. As I recall, there is some slop in the fit, especially there at the front when using the O-rings as the "keeper". I have replaced all the O-rings with the old-style locking screw/knob and there is no wiggle or wobble of the adjustable stop. I think the difference is that with the locking screw, the opposite side of the stop is firmly pushed against the shaft, preventing any wiggle. With the O-ring, that doesn't happen, so in addition to being able to rotate (and change the projection), it can also wiggle side to side near the front.
Rick