Bought a used Smith model 66 years ago. Great gun but I was having light strikes randomly (similar) to what you are describing.

Turns out the original owner had backed out the strain screw quite a bit and ground it off flush. I can only surmise it was right on the cusp of reliable at that point. It would set off most primers but would light strike randomly. My guess is its not simply ammo batch to batch or manufacturer to manufacturer but literally round to round in terms of mass production.(the primer pocket is a little bigger on this one, a little smaller on that one we used a different primer vendor and you got rounds with the different primer etc. etc. etc. basically minute differences that can tolerance stack.

Long story short I replaced the strain screw, tightened it up and all was back too good to go.

I simply think, like was said above, your on the cusp of totally reliable. You may have some internal wear, drag that slows down certain hammer drop or the cylinder is slightly off center on certain rounds etc. etc. Something minute that is tolerance stacking.

Go back to stock, run it stock for a thousand rounds or so and try the MCarbo parts again.

I had something similar happen to me with my S&W 625. I installed a lighter hammer mainspring and started getting light strikes in double action but not single action. I finally figured out that in single action the hammer comes slightly further back than it does in double action, and that small amount of travel was the difference between a light strike and a good primer hit.

I ended up putting the regular mainspring back in and replacing the triggr return spring with a reduced power one to take some weight off the trigger. I also left it cocked in single action when I wasn't using it and that broke the stock spring in to the point that I got the feel I was looking for.