A flipper loses strength over time, playing or not, until it stops working.

Flipper gets weaker the longer the game is on

Cause: Dirty or failing optos on the cabinet flipper board, or failing LM339s (Fliptronics U4/U6, WPC-95 CPU U25/U26).

Checks

• Clean the optos.
• Swap the left and right flipper boards (both plugged in) and see if the fault moves.

Fix

• Replace the opto, or as a last resort the LM339s.

Systems

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

Related tips

• Fliptronics: flipper coil gets hot after a while of play
• Two LM339s on the Fliptronics board read the flipper optos
• Weak or dead flippers from failing LM339s on the Fliptronics board
• Resets remain after rectifiers, caps and connectors are done
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• A dedicated (direct) switch does not work
• Switch matrix chips on the CPU board
• Row or column "ground short" errors

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