Quoted from Pinballomatic:Monkey drop issue is usually because the monkey goes a shade too far up before he stops, thus the ball drops just a little too high and hits the rounded ball guide and spins back off. Adjust the top monkey microswitch JUST a little so that the monkey stops just a bit sooner and the ball will fall more directly down into place. Also make sure that the small black weatherstrip piece is in the right spot. Bring it away from the rightmost edge so that it kills the balls momentum and keeps it in the "basket" better.
This was my exact issue as well. As you suggested, I bent the micro switch a "tiny" bit and now my ball is landing perfectly every time. Thanks!