One pop bumper fires unreliably when the ball hits its skirt, while the coil and bulb are fine.

Pop bumper fires only now and then: clean and gap its skirt switch

Cause: Dirty contacts and a poor gap on the leaf switch under the bumper skirt.

Checks

• Watch the bumper's switch in switch test while tapping the skirt from different sides.

Fix

• Clean the switch contacts and set the gap so a light skirt tilt closes it.
• These low-voltage switches have gold-plated contacts: clean them gently (a business card or a contact-safe cleaning strip) rather than with sandpaper, which can strip the plating.
• Bend the blades carefully: a blade bent too far the wrong way may not bend back.

Systems

• Williams System 11A

Games

• Banzai Run

Related tips

• Catapult sometimes throws the locked ball early: clean or replace its switch
• Lane change and shooter lane switches don't register: failed column driver Q42
• One switch triggers an unrelated switch (phantom closures)
• Matrix burned by the lane-change switch touching the flipper EOS switch

Source: Pinball Repair help group!! (Facebook group)