A stepper misses steps up or down.

Stepper will not step reliably: sticky activator arms

Cause: Gummed pivot points on the step-up or reset/step-down activator arms.

Checks

• Move each activator arm by hand and feel for stiffness.

Fix

• Strip and clean sticky pivots with alcohol, then put a little Teflon gel on metal-to-metal pivots.

Systems

• Electro-mechanical

Related tips

• Game will not work: stepper unit binding
• Stepper step-up arm returns weakly
• Repair a broken or worn stepper wiper arm
• Gottlieb credit unit will not step up or down
• What stepper units do in an EM
• Ball count stepper won't advance: pawl arm frozen in old grease
• Kinds of stepper unit
• Faults that point to a stepper unit

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