The double bonus doesn't collect on both sides. Watching the bonus unit reset, it skips over gear teeth.

Double bonus not collecting: bonus stepper mainspring overwound

Cause: Someone had wound the stepper's mainspring too tight (probably to cure an earlier sticky unit). Worn gear teeth added to it.

Checks

• Clean the stepper so it moves freely.
• Watch it step and reset; if it skips teeth, check the mainspring tension.

Fix

• Back the mainspring off by one turn.

Systems

• Electro-mechanical

Games

• Grand Prix

Related tips

• Game stuck on player 1, ball 1: broken wire on the ball index relay
• Game will not work: stepper unit binding
• What stepper units do in an EM
• Kinds of stepper unit
• Game ends at the wrong time: stepper wipers off their rivets
• Faults that point to a stepper unit
• Cleaning and aligning an EM stepper unit
• Player unit steps up or resets, but not both, after new springs: refit the originals

Source: Pinball Repair help group!! (Facebook group)