I had this problem and solved it by loosening the screws on the mech under the playfield, repositioning it until it was in the proper position. You can also bend the sword a bit, but only after making sure your machine is level. The ball can definitely roll off the sides of the sword easily if things aren't level.
Finally, since my game was being used in competitions and I wanted to bulletproof it, I added a thin strip of weather strip foam along the left side of the sword to ensure that it would never pop out. But adjusting the mech's position solved the issue 100%.
Ronkonkoma, NY
