Start is pressed but the backglass stays on ball 3. Pushing the ball count reset coil by hand resets it correctly.

Bally Nip-It: ball count unit doesn't reset at start

Cause: The reset coil gets too short a pulse because the coin relay isn't held long enough. The coin relay holds through its own hold switch or a credit relay switch, then score motor switches 8F and 10F; here the credit relay switch was dirty and arcing.

Checks

• Watch the ball count reset coil: if it twitches at start, the pulse is too short.
• On the schematic the reset coil shares its switches with the coin unit step-up coil and the total play meter: motor switch 3A and a coin relay switch. If the play meter does advance, the reset relay make/break switch is fine.
• Jumper the coin relay switch closed; if the unit then resets, look at what holds the coin relay in.
• Check motor switches 8F and 10F, the coin relay hold switch, and the credit relay switch while pressing start; look for arcing.

Fix

• Clean and adjust the arcing credit relay switch.

Systems

• Electro-mechanical

Games

• Nip-It

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