One or both flippers are weak with good switches.

Weak flipper: rectifier or worn parts

Cause: Worn plunger, sleeve or link, loose bolts, or rarely an open diode in the flipper bridge rectifier (weakens both).

Fix

• Rebuild worn mechanical parts and tighten bolts.
• Replace the bridge rectifier if both flippers are weak and it tests open.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C
• WPC Alpha-Numeric
• WPC Dot Matrix

Related tips

• Pre-Fliptronics: weak flipper
• One or both flippers weak
• Weak flippers from pitted EOS switches
• Weak flipper: dirty end-of-stroke switch
• Contact files only on flipper switches
• Flipper rebuild part numbers for System 11
• Every System 11 flipper has an EOS switch and coil diodes
• Sluggish flipper with nothing binding: EOS switch not closed at rest

Source: Pinball Machines: How They Work & Troubleshooting, Norbert Snicer