One or both flippers are weak.

Fliptronics and WPC-95: weak flipper

Cause: Usually mechanical wear (mushroomed plunger, binding). Then dirty or marginal cabinet flipper optos, an EOS that closes too early, LM339 comparators (U4/U6 on the Fliptronics board, U25/U26 on the WPC-95 CPU board), poor flipper power connections, or the Fliptronics board bridge BR1 (both flippers weak).

Checks

• Check for about 1/16" vertical play in the flipper.
• EOS should close right at the end of the stroke.
• Swap the cabinet opto boards (both must be plugged in); a fault that moves is the opto.
• Game on, button pressed: the opto board SW1/SW2 pin should read under 0.7 V (under 1 V is acceptable) to ground; higher means a dirty or failing opto.
• Inspect J902 and J907 on the Fliptronics board (J119 and J120 on the WPC-95 driver board).

Fix

• Rebuild the flippers.
• Clean the U optos with glass cleaner on a cotton swab, make sure the activator clears fully, replace a marginal opto.
• Replace the LM339s if nothing else helps.
• Repair connector joints, and check or replace BR1 on the Fliptronics board.

Systems

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

Related tips

• Two LM339s on the Fliptronics board read the flipper optos
• Weak or dead flippers from failing LM339s on the Fliptronics board
• Fliptronics: flipper coil gets hot after a while of play
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Resets continue although the bridge and diodes test good
• Resets remain after rectifiers, caps and connectors are done
• Lamp matrix faults with all lamp transistors good
• A dedicated (direct) switch does not work

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