One or both flippers are weak.

Pre-Fliptronics: weak flipper

Cause: Usually wear: a mushroomed plunger dragging in the coil sleeve. Also a misadjusted or pitted EOS, pitted cabinet switches, bad J109/J110 connections, or an open diode or cracked joints in the coil bridge BR3 or its capacitor C8 (this weakens both flippers equally).

Checks

• Lift the flipper from above: there should be about 1/16" vertical play; no play means binding on the playfield bushing.
• Check the EOS opens by only 1/16" to 1/8".
• Inspect J109 and J110 header joints on the driver board.

Fix

• Rebuild the flippers.
• Reset the gap by moving the pawl on the shaft under the playfield.
• File the EOS and cabinet switch contacts.
• Repin J109/J110 or resolder cracked headers.
• Replace BR3 or C8 if faulty, and add bridge-to-cap jumpers.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix

Related tips

• Pre-Fliptronics games need a driver board with the flipper relay
• Pre-Fliptronics: a flipper does nothing
• Pre-Fliptronics: flippers work even with no game running
• Pre-Fliptronics: flipper lane change does not work
• Flipper buttons not seen by the game (pre-Fliptronics)
• Pre-Fliptronics WPC flipper stuck up with power on: failed driver TIP102
• Pre-Fliptronics flipper coil wiring
• Weak flipper: rectifier or worn parts

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