The game reports a missing ball.

"Pinball Missing" message

Cause: A ball is lost or stuck, or a trough, outhole or shooter lane switch is misreading.

Checks

• Count the balls in the game against the number it should have.
• If all are present, test the trough switches, outhole switch and shooter lane switch.

Fix

• Recover the ball or repair the faulty switch.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C
• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• "Pinball Missing" message
• Balls hang in the trough: divots in the trough metal
• Old and new WPC ball trough designs
• Opto ball troughs from Indiana Jones on
• Two balls served at start and the lock spits its ball out: count the balls
• Full ball trough opto test in switch edge test

Source: Pinball Machines: How They Work & Troubleshooting, Norbert Snicer