Presuming the revolver is in fine or excellent condition other than this issue ... is there any describable fouling (smoke ring, etc) at the exit ports of the cylinder and / or the build up at or around forcing cone ? Mind, old lead foul rings might not be that easy to distinguish against the Stainless back drop.
The slightest build up here will give you the problem you manifest.
Clearances between cylinder and forcing cone should be "tight" but not tight enough you cannot sequence the action. For example, Dan Wesson's clearances are extremely tight and precise here.
If so, I suggest:
1. Remove the cylinder, then remove the extractor
2. use 500 wet / dry sandpaper with some WD 40 behind a small flat block of hard wood (e.g. 1/2" x 1/2" x 4" long) against the flat rear face (exit holes and surrounding area) careful to ONLY lightly sand the flat surface ... then carefully clean the fouling rings. If you think you're comfortable with it ... you can use a very fine India stone cut this same dimensions. If you've never used India stones don't venture this process.
3. Bronze / Brass brush and Hoppes #9 across the face of the forcing cone
The bronze or brass brush will leave a copper-gold halo on the stainless that will wipe, wash or lightly buff off when finished.
4. Reassemble, lubing working parts in the process.
Yes, some will say the Hoppes might do long term damage to the stainless, nickel, blue etc. Just make sure you thoroughly neutralize or flush well with WD 40 until you've purged all remnants of the Hoppes.
I personally think Hoppes should make a cologne for men "Hoppes #9 ... for Men"