After the step-up coil fires, its arm returns slowly or not at all, so the next step is missed.

Stepper step-up arm returns weakly

Cause: The step-up return spring has lost tension, or an over-wound clock spring is overpowering it.

Checks

• Check clock spring tension first.
• Watch the step-up arm return after each pulse.

Fix

• Cut a few coils (about 1/4" to 3/8") off the return spring and bend the new end loop out to hook on again.

Systems

• Electro-mechanical

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Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part One, Clay Harrell (pinrepair.com)