Have you rebuilt the mechanics on the flippers yet - new plungers, pawls, bushings, springs etc.

It is almost always mechanical/electrical issues that make them feel weak. It is very unlikely you need a new coil unless someone replaced them with the wrong part.

Start here if you think it's your flippers:
https://www.marcospecialties.com/pinball-parts/BFKIT74

pasted_image (resized).png

Clean and free doesn't really mean a lot in this case. All of the mechanical parts develop wear and you can lose significant power if there is play in the bushings or in the connection between the plunger and the pawl/crank, or has worn plunger or sleeves.

Could also be dirty/bad EOS switches if they are normally closed style.

If you have 2 of the machines, why does one have weak flippers and the other doesn't? It's not the coils. Coils either work or they don't. They don't loose power over time. The machines' flippers are different because of different amounts of wear / maintenance on the flipper parts.

Have you rebuilt flippers before? It's easy and the results can be stunning on old games like these.