Yep! Common problem with the mode start as I found out. I fixed mine by doing a couple of things.
 Elongated the screw holes on the metal stopper so I could move it further back.
 Installed a Cliffy protector.

These didn't seem to help to much.

 I think what helped the most for me was I pulled the metal stopper out and noticed the face that the ball hits was just slightly bent from repeated ball strikes. Squeezed it in a vice to flatten it out and re-grained the face that the ball strikes. It worked 99% of the time after that. Its been 12 months now and it has just started to reject balls so probably it has started to bend again and needs redoing.