I believe there is a bonus stepper unit ( probably on the underside of the playfield) that scans the bonus. On it, will be a bakelite part containing wipers (snow shoes) that skate across the river heads. Usually, there are stranded jumpers that either join nearby wipers or connect a wiper to a grounding lug under one of the three adjusting screws that mount the wiper. Check for breaks.

If it has a trip bank relay mounted to the underside of the playfield, check to make sure the bank is fully seated in the mounting slots and the wing nuts are tight. A slight twist of those units can cause contact make issues on the relays.