Heed Bill’s advice, “lost motion”/slop could be the issue here. One more simple thing to try. Have an assistant hold that vertical shift detent out of the way against its spring pressure. Now spin the prop and shift into reverse to the engine its travel in reverse. Does the shift lever’s pin travel way past where the vertical detent rod would normally position it in reverse? If so, then I would suspect lost motion/slop as the issue.
