Replace the coil sleeve if you haven’t already done so. In rare cases I have had issues with the plunger getting gummed-up and sticky, fixed by cleaning with acetone or naphtha (remove the plunger to clean and don’t reinstall until all solvent has thoroughly evaporated).

Make sure the flipper is set to the correct height above the playfield (should be about a mm of clearance between the flipper bat and playfield bushing - use a business card to gauge). The playfield bushings can wear out, but I’d be surprised if they did so in just 700 plays (not hard to replace in any case).

Check the coil stop; I have seen mushroomed stops cause this symptom, and it isn’t always obvious when it happens.

Test by hand - the flipper should move very smoothly, with the spring providing the only resistance.