On some ball drains nothing happens: the bonus unit and score motor never start, so the next ball is not kicked out. It tends to happen every other ball, and tapping the flipper buttons quickly can get the game going again.

Next ball sometimes not served after a drain: C relay path switches, then new rubbers

Cause: When the first-ball switch is open, the outhole path runs through the C relay to step the bonus unit. A switch in the circuit that energizes C (here QB1) looked closed but had poor or no contact. After that was cleaned, the fault came back now and then until the old, dried rubber rings were replaced; a switch held or bounced by a hard rubber was the likely culprit.

Checks

• With a ball in the outhole, see which relay fails to pull in (here C); pulling it in by hand lets the cycle finish.
• Check each switch on the schematic path that feeds that relay coil with a meter while it is closed, not just by eye.
• If the fault changes with vibration (flipper taps), a switch there is marginal: clean, gap and reflow its solder tabs.
• Look for old, hard rubbers near playfield switches that could hold or bounce a switch.

Fix

• Clean and adjust the switches in the C relay circuit, including QB1.
• Re-rubber the game with a fresh ring kit.

EM games run 120 V on some circuits; unplug the game before working in the backbox or on the relay bank.

Systems

• Electro-mechanical

Games

• Hot Shot

Related tips

• Game starts but flippers are dead and nothing scores: dirty relay switches
• Game Over relay trips again right after start: dirty Lock relay switch
• Game resets but the outhole relay never fires: feed wire hanging by a strand
• Ball count stays at 1 after each drain: dirty secondary trough switch
• Game stuck on player 1, ball 1: broken wire on the ball index relay
• Score motor runs on at game over: end game relay not pulling in
• Score motor keeps running at start and the game never starts: clean the ball count unit
• Start button adds players 1 to 3 but never the 4th: adjust the coin unit end switch

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