Rob - it is a fiddly board to get right. I prop my playfield up, and turn the potentiometer to it's highest sensitivity and it should register when you place the ball in the vicinity. The light on the board should be on (from memory). This means that the switch will ALWAYS detect the ball.
Then turn the potentiometer the other way, and when the light JUST switches from on to off, it should detect the ball correctly.
