A stepper steps up sluggishly.

Stepper coil plunger drags in its sleeve

Cause: The plunger or sleeve was oiled and has gummed up, or the plunger end is mushroomed.

Checks

• Pull the plunger and inspect it and the sleeve.

Fix

• Clean plunger and sleeve with alcohol; never lubricate them.
• File off a mushroomed plunger end.

Systems

• Electro-mechanical

Related tips

• Game will not work: stepper unit binding
• What stepper units do in an EM
• Kinds of stepper unit
• Relay banks and their reset coil
• Faults that point to a stepper unit
• Stepper cleaning, part 4: refit and test
• Ball release coil stays on: burnt wires behind a stepper plate
• Cleaning and aligning an EM stepper unit

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