Flipper is weak even with new parts and a correct EOS gap.

Flipper still weak after a rebuild: wiring and contacts

Cause: Dirty or pitted EOS or cabinet switch contacts adding resistance, cold or cracked solder joints at the EOS switch or coil lugs, or solid-core wire between coil and EOS that has broken inside its insulation (common on Williams).

Checks

• Inspect and reflow the solder joints at the EOS switch and coil.
• Flex the coil-to-EOS wires and look for solid-core wire.
• File the EOS and cabinet switch contacts.

Fix

• Resolder bad joints.
• Replace solid-core coil-to-EOS wires with good stranded wire.

Systems

• Electro-mechanical

Related tips

• Flippers still weak after a rebuild kit: clean the new EOS contacts
• Gottlieb EM flipper rebuild parts
• Flipper EOS switch: gap, checking and cleaning
• Zipper flipper rebuild parts and stronger coils
• Rebuilding an EM flipper
• How an EM flipper coil and its EOS switch work
• Rebuild EM flippers before fitting stronger coils
• Weak flippers on an EM: adjust the end-of-stroke switch before buying coils

Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part Two, Clay Harrell (pinrepair.com)