I had a problem like that with mine. Make sure the flipper has about a little play between the flipper bat and the playfield. If the bat is too tight to the surface it will be extremely weak due to binding.

Coils either work or they don't.

At a minimum, you need a new coil sleeve...a new flipper bushing could only help as well.

But you just might as well go ahead and buy a rebuild kit and rebuild both flippers while you're up there. The bushings are sold separately.

That way you'll know exactly where you're at by having a factory-fresh starting point.

Might want to rebuild the three lowers while you're at it.

Check to make sure the bats aren't cracked including looking underneath at the ribs and flexing them.

Personally, I automatically rebuild the flips/replace the bushings after getting a new game (unless they're low-play HUO, or I know the PO has recently done the work).

I rebuild the pops & slings too...but that's another story.

Check the springs. I would swap them between the flippers and see if it follows the spring or stays with the coil.

Does the flipper lag at all when you make a shot? Does it seem to come back to rest slower than the other flipper? If that's the case it could be the coil sleeve.
If you take it off again, check the wires going to the coil and make sure they are soldered on well.

This is what I remember being the problem with my FGY when I owned it and had the issue. I didn't even need to swap the spring, just played with it and broke it free. I say look there.

If you have cleaned it well, replaced things I would rebuild that flipper as NM has stated. Maybe compare the voltage to each coil and see if there is a signficant difference. Like I said before, with the game off, you should be able to move the flipper slightly up and down.
On my game, I can not always make the lois shot from the left flipper 100% of the time but I can make the shot fairly well. If you can't make it at all, it's either a mechanical or electrical problem. It might just be an electrical problem. Don't want to stare you one way or the other. Sure it is something simple (it usually is).