I just went through this with a Taxi I rebuilt a couple of weeks ago. I did everything you can imagine.. Positioning the shooter in numerous places, building up the cracked ramp holes with epoxy to make sure it's held tightly in place when screwed down, bending the switch to various positions, polishing the hell out of the ramp to make sure it's slick, shimming the entry point of the wireform, etc and nothing made a difference at all.
Then I took a popsicle stick, broke it in half, and put one half under each "hook" that the playfield connects to the lockdown receiver with, slightly raising the position of the playfield relative to the shooter. The shooter couldn't be adjusted down this far on its own -- I had already moved it as far as it would go. Now when I plunge, the ball almost makes it all the way through the numbers twice. Previously it would only get to the 4th or 5th.
Pretty ghetto hack, but it did the trick when nothing else worked! :\