Has the flipper shaft been replaced lately?
I fitted new shafts from PBR on my Jumping Jack and had the same problem.
The recessed section on the shaft (Where the grub screw grabs)has not been machined far enough down the shaft,so the screw wont bite.
Hope this makes sense.
Millsy

+1
You could always file a small flat where you can see the marks from the locking screws.
This will give the screw a nice flat surface to bite onto, instead of two high points on a diameter.