Quoted from Rarehero:
Absolutely wrong. The ball just barely tripping the lower opto will release the magnet ball & start your multiball. Getting a dead on whack is fun, but you don't have to do it. If you need a video for proof, I'll make you a video for proof. I own the game, and this is how it behaves. I've played another ST where the ball got stuck on the magnetized ball every single time and you had to wait for it to time out. That's abnormal behavior and there's something wrong with the lower opto.
I can confirm this as having worked on one with a bad lower opto and the magnet ball will not release no matter how hard you hit it and you end up with 2 balls stuck to the magnet. OP might have a dieing lower opto, we had (or have really, still needs to be fixed) a DEAD one. STLE