Open the game up. Physically inspect the flipper underneath. Compare with the good flipper. Several physicals things can affect a flipper.
Coil sleeves are cheap to replace
The rod in the coil should feel smooth going through the coil and sleeve. Inspect the plunger rod for abrasiveness or mushrooming. A file usual cures that.
The plastic epic connecting the plunger to the flipper wears out - the hole it links to gets bigger, looser, sloppier
The coil stop may simply be loose
Consider the vids guide to flipper rebuilds here.
Page 43 of the Whoa Nellie manual covers the flipper assemblies. You can download that from Stern if the
Pabst manual is lacking.