Dead simple Neil!
Too much slack between the thrust washer and the shaft tube at the prop end.
Any excessive movement there when going ahead, caused by the highly desirable thrust of the prop😉, is mirrored at the coupling end. Hence your 'traffic' jam!🤔
You can easily check the amount of movement. Simply pull the prop back as far as you can and measure the gap between the locknut/thrust washer and the end of the shaft tube.
Reduce it to about 10 thou.
If you run out of thread at the prop end to do this then you will need to adjust the coupling end as well by moving it closer to the prop tube.
If you can use a dished or 'crinkle' type thrust washer, they have more resistance to being pushed forward and have a spring back effect.
The above adjustment should be done with the coupling up against the inboard thrust washer.
That should leave more than enough movement in the coupling without jamming.
(I enjoy a good Jam Session on the drums though!😁)
Hope that sorts things out.
Cheers, Doug 😎
PS The coupling cannot 'move away from the prop shaft' (unless you forgot the Allen screws😮), it can only move away from the shaft tube!