One opto switch never registers while other optos in the game work.

Opto does not work: check 12 V at the transmitter connector

Cause: No 12 V or no ground reaching the transmitter LED (broken wire or connection), a cold solder joint at the opto, a burnt-out transmitter, or a broken lead.

Checks

• Do not trust a reading on the LED legs themselves; with the LED in circuit it shows only about 1 V.
• Unplug the opto and measure for 12 V DC at the harness side of the connector. The transmitter is usually on black and gray wires; the orange and gray pair goes to the receiver.
• If 12 V is there, check whether the transmitter is lit with an infrared sensor card, or with a digital camera that has a live LCD screen.

Fix

• No 12 V: find the break in the 12 V or ground wiring to the transmitter.
• Resolder cracked joints on the opto's little board and reattach any wire broken off by vibration.
• 12 V present but no infrared light: replace the transmitter LED.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Opto still dead after replacing both sides: the new transmitter can be bad
• Testing optos in switch edge test
• Gumball enter opto (switch 87) never registers: replace the LM339
• Idol optos missing from switch test: use idol test, then fix connectors
• Flipper cabinet opto switch boards
• Where WPC games use opto switches
• Telling an opto transmitter from a receiver
• Bookcase opto still dead after a new receiver: the replacement part was bad

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Three, Clay Harrell (pinrepair.com)