Quoted from Bumper:

Is a known issue, they fixed it with later models using finger guards. The metal between the targets makes it less likely that the ball gets stuck, this part is unobtainable, I hope that someone designs a 3D printed one.

Quoted from frenchmarky:

I have an original 81 model and the ball has never once gotten stuck between the downed targets since I've owned it for 30+ years. And I don't see any finger guards on it. You say the targets look as if they are sitting too low. On mine the bottom shelf of the drop target unit is adjustable up and down via slots in the shelf where the four screws mount, so you can raise/lower the 'down' position of the targets, you might check that out on yours. If the targets are too low I can see where the ball could easily get stuck.

Quoted from frenchmarky:

If your targets really are too low and that is causing your issue, you could just remove the shelf and add a long thin shim of whatever material (metal, layers of thick tape etc.) to raise the targets. Me? I'd just lengthen the shelf holes vertically a little with a dremel so it's like the adjustable version and I'm not messing with different thicknesses of shimming and then attaching it.