The Ball Return (outhole kicker) is activated via a normally closed switch on BX, and normally open switches on O, motor 4C, and motor 1A.
So when the ball drains, the ball return switch should close, which activates the O relay, which has a switch that closes and causes the score motor to run, which then will cause the score motor switches to closes and energize the outhole kicker (Ball Return).
So the first thing to check is whether the O relay is activating when the outhole switch closes. If not, then there's a series of about seven switches to check in the path (including the outhole switch itself). You can see this path on the schematic at around 19-E.
If it does activate, then does the score motor run? If not, then check the switch on O.
If the score motor runs, then check the BX, O, and motor 4C and 1A switches that activate the Ball Return coil.