You'll need to look at the switches that provide power to the ball return solenoid. Looking at the portion of the schematic HowardR posted, examine the following switches:

O relay switch - brown/red wire and orange/red wire (normally open [NO])
BX relay switch - orange/red wire and yellow/black wire (normally closed [NC])
Score motor switch 4C - yellow/black wire (normally open [NO])

Make sure theses switches are clean and adjusted properly.

If these switches look good, then you'll need to see if the NO switches in the circuit are closing when the ball enters the outhole. Is the O relay being energized when the ball is in the outhole?