Quoted from awesome1:
I also have messed around a bit with the auto plunger and haven't had permanent success. I try to manual plunge most of the time, but it also runs into problems from time to time.
If it makes you feel better (it won't, but... ), the autoplunger isn't an AIQ problem, it's a Stern problem, and it's been one for a very, very long time. It stems from the nature of the design. There is simply too much slop when you take into account the tolerance stack on all of the pivot points and moving parts. Most games it doesn't matter if you get the occasional (or often) weak plunge because the design isn't demanding. Unfortunately games like AIQ, JP, BK:SoR (prem/LE), and even as far back as the original Avengers struggle. Pretty much anything that needs to go up some sort of ramp or major incline during it's launch is going to have potential issues, while games with a nice flat, simple plunger lane likely will not (since a weak plunge will still make it).
Making sure the forks are aligned, adding washers or e-clips (or felt bumpers, misc other things people have tried) to take up slop in the assembly, putting washers or spacers under a portion of the shooter lane to help guide the ball, etc can help give it a better fighting chance, but the odds of ever getting 100% are low. The entire mechanism would have to be redesigned or manufactured to WAY higher tolerances to perform consistently at full power. Everything else is a band-aid. Sure, some people will get lucky with parts on the right side of the tolerance band, but some will not.
I am pretty lucky with my AIQ, it performs at about 80%. I still need to tweak my forks and I can probably bring that up a little. But I feel for those that have it worse. I have fought with other games, and it isn't fun when there is really nothing you can do about the root problem. But you can minimize it as best as you can and try not to let it ruin the game.
As was mentioned, Soul Gem is the place it can really hurt you most. For that you can try manual plunging before the auto plunges. Not ideal, but it might help in a critical moment.