If you lift up the playfield, there should be a switch that is normally open on the right front under the playfield. This switch advances the ball count unit when the ball rolls over it after it is kicked from the outhole, but before it hits the plunger lane. It might need to be cleaned & gaped properly. You should be able to flick the ball with you left index finger from the outhole & stop it with you right index finger before it gets to the plunger lane. This is a good test of the tension on the switch blades & also you should see some movement from both blades.