Your bullets may need to be seated deeper. The ogive/shoulder may be just kissing the rear portion of the cylinder throat, which will prevent the cartridge from chambering fully in the cylinder. The protruding casehead may be binding on the rear wall of the frame.
I offer this, as it happened to me this past winter. I shoot in both conventional and centerfire pistol winter leagues. I was doing some load developments with my S&W 14 and S&W 586 with regard to bullet seating. On one load, I encounted "binding" when cocking the hammer on 2-3 times/cylinder. I opened the cylinder and pressed the lead wadcutter-loaded cartrides into the cylinder, and felt it chamber fully-then tried shooting again. No binding. I just seated that load out a little too far. I wanted to have te leading edge of the wadcutter to just touch the rear portion of the throat. I apparently seated it a few thousanths too long.
Subsequent loading/shooting with a touch deeper seated bullet has resulted in "normal" (non-binding) action operation.
Try seating your bullets deeper. What are you shooting for a load? Cast? Jacketed? Factory? Handloads? What bullet? If they are handloads, you may have to trim your cases to a uniform length too, if everthing else is "right," to allow a loaded cartridge to be "shorter."
A cylinder gap of .004"+ should be fine. There may be some non-uniformly deposited gunk on the front of they cylinder. Use a brass bristled brush and cleaning solvent to clean up the cylinder face.
I'm putting my money on the cartridge being loaded a skooch too long, and getting caught on the rear shoulder of the cylinder throat.