When your N-frame revolver was made can make a big difference. Prior to 1962 or thereabout the cylinder stop (the part that engages a notch in the cylinder and locks a chamber so it’s concentric to the bore) was tensioned with a coil spring and plunger that was blocked by a screw mounted in the front of the frame, and above the trigger guard. Do not confuse this screw with the front side plate screw.
Starting in approximately 1961 the design was changed. The plunger, spring and screw were eliminated and replaced by just a spring that was nested in the cylinder stop on one end, and a cutout in the frame on the other. While this system was less expensive it had some drawbacks, one of which was that the spring could become kinked or bent when it was reassembled, and then thereafter the stop might move in strange ways.
My earlier post presumed that you had the later system rather then the former, and may – or may not have been in error. This is another instance where a good photograph is worth a thousand words.
Anyway I now think that in addition to the above, part or all of your problem may be too much Cerakote in the cylinder's notches. In the old days of cheaper labor costs S&W assemblers took pains to insure the fit between the cylinder stop ball (that part of the cylinder stop that sticks up through into the cylinder window and goes into the notches in the cylinder) were a snug fit. A coat of Cerakote could have made the fit too tight.
Remove the side plate, hammer and mainspring. Leave or reassemble the rest. Then pull the trigger while watching what the cylinder stop is doing. A light bulb may go on.