Quoted from pavelump:
I struggled with this issue for a long time. I’d adjust, file, wax, you name it. I’d get it dialed in and then it would start acting up. I finally replaced the ball guide adjustment screw with a bolt and a t-nut. Since then, I’ve been able to really lock that guide down, and so far I’ve been about at a 95% success rate. Certainly a clean playfield and lack of burrs on the end of the guide are really important too.
+1, having that ball guide locked down is the critical step. Since doing so, I've been 95%+ for over 1,000 games.