An extra (upper) flipper is weak while the lower one on the same side is fine.

Upper flipper weak on a multi-flipper game

Cause: On some games the upper flipper is fired by a second, normally open switch on the lower flipper's EOS stack that closes as the lower flipper reaches full stroke; that switch is dirty or misadjusted.

Checks

• Look for a three- or four-blade EOS stack on the lower flipper and trace the extra switch to the upper coil.
• Jumper that switch; if the upper flipper gets stronger, it is the culprit.

Fix

• Clean and adjust that switch until the upper flipper is strong.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7
• Williams System 9
• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C
• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95
• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200
• Bally 6803
• Gottlieb System 1
• Gottlieb System 80
• Gottlieb System 80A
• Gottlieb System 80B
• Gottlieb System 3
• Data East CPU Version 1/2
• Data East CPU Version 3
• Data East/Sega CPU Version 3b
• Capcom
• Electro-mechanical

Related tips

• Flippers can't get the ball up a steep playfield
• Flipper weak or noisy: worn fibre link
• Weak, chattering or buzzing flipper: end-of-stroke switch
• Flipper weak because the return spring is too strong
• Flipper bat slips on its shaft or wobbles
• Upper flipper weak or dead when the lower one is
• Replacing a flipper coil
• Flipper weak: worn plunger, coil stop or sleeve

Source: Pinball Machine Care & Maintenance, Bernard B. Kamoroff (Bell Springs Pinball Alley)