The flipper is weak.

Weak flipper: dirty cabinet button

Cause: The cabinet flipper button contacts are poor.

Checks

• Jumper across the button with a test lead; if the flipper is then strong, the button is the problem.

Fix

• Clean, adjust or replace the button switch.

Systems

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

Related tips

• One flipper does not work at all
• Flipper sticks in the up position
• Pre-Fliptronics: weak flipper
• Flipper dead on a System 11 game
• One or both flippers weak
• Flipper button sticks
• Weak flippers from pitted EOS switches
• Pre-Fliptronics: a flipper does nothing

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