Quoted from John_I:Mostly it seem to be over scoring. For instance I can knock all 15 targets down and get 17,000 in bonus.
I put an answer out on RGP, but this seems to be a little more specific, so....check to make sure your stepper is actually stepping each time the coil fires. The coil will continue firing until you get the the 15th position switch. You may just need a slight adjustment in stroke or you may have a coil stop issue.