One or both flippers lack power.

Weak flippers

Cause: Worn parts (a mushroomed plunger dragging in the sleeve is classic), binding at the playfield bushing, pitted EOS or cabinet switch contacts, or an open diode or cracked joint on the flipper bridge rectifier (which weakens both flippers equally).

Checks

• Lift the flipper bat: there should be about 1/16" of vertical play; none means it is binding (adjust the pawl on the shaft from below).
• Inspect EOS and cabinet switch contacts.
• Check the bridge rectifier and its lugs.

Fix

• Rebuild the flipper.
• File the tungsten EOS and cabinet switch contacts, or replace the switches.
• Replace the bridge if faulty.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• Where flipper power comes from
• How flipper power is routed by generation
• Power at the flipper coil but no flip
• 50 volt flipper power supply on the last three System 7 games
• Flippers dead: fuse and coil voltage
• Flipper fuse blows
• Measure a flipper coil
• Three flipper mechanism generations

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)