Quoted from KnockerPTSD:
The biggest flaw in my game is still the bloody auto plunge. It fails about 80%. Prongs are lined up nicely. Really takes the momentum out of the gameplay and messes up the Soul gem
That's a bummer. Did you try placing a washer under the left fork on the metal "ramp" at the end of the shooter lane (similar to what many people do on JP)? My shooter lane was at about 75% out of the box. I first tried a very thin washer and that didn't change much, but a slightly thicker washer worked great. I have played probably 20-30 games since doing that and haven't had a bad plunge yet. That's not to say it won't happen...the auto plunger mech on these simply aren't capable of being perfect, but I was surprised how much of a difference it made on mine.
I would also make sure your mech is tight and not binding up on anything. If it's failing 80% of the time, that seems like a bigger issue than the typical Stern plunger. Maybe it is slightly misaligned? I would try loosening the mounting screws on the whole mech and see if there is any slop or if it can be shifted slightly. Maybe it's aligned when at rest, but is actually pushing the ball at a slight angle? Donno...just throwing out ideas. Best of luck!