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.
https://pinside.com/pinball/forum/topic/vids-guide-to-upgradingrebuilding-flippers
Page 43 of the Whoa Nellie manual covers the flipper assemblies. You can download that from Stern if the
Pabst manual is lacking.