The flipper button shows no change in the switch edges test, so the flipper does nothing.

Fliptronics: flipper button does not register in switch test

Cause: A dirty or failed cabinet flipper opto board, or a warped plastic opto activator that no longer clears the U-shaped opto. Rarely, failed input chips U5 (74HCT244) and U2 (74HCT374) on a pre-WPC-95 Fliptronics board, or a poorly seated CPU-to-Fliptronics ribbon cable.

Checks

• Check the opto board and activator by the flipper button.
• Reseat the CPU-to-Fliptronics ribbon cables, taking care to fit them correctly.

Fix

• Clean or replace the opto, replace a warped activator, or replace U5/U2 on the Fliptronics board.

Systems

• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• How Fliptronics flippers work
• Fliptronics: flipper will not hold up (flutters)
• Fliptronics: flipper coil gets hot after a while of play
• Flippers play fine but buttons do nothing in video mode or initials entry
• Fliptronics flipper does not work: test the flipper opto board
• Fliptronics and WPC-95: weak flipper
• Upper flipper sometimes weak, then buttons dead after cleaning: replace the flipper optos
• One flipper dead and its button fails switch test: dirty flipper button opto board

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