Load the cylinder with dummy rounds. (A real case with a real bullet in it. NO powder or primer.
Then do your SA test. Sometimes the cylinder lock-up is so close that the added "drag" on the recoil shield will add enough "delay" in the cylinder rotation to allow the cyl. stop to drop in properly to the cylinder notch.
Or just cock SA with a finger tip touching the cylinder.
Or as mentioned above pull the hammer back just a bit faster than slow.
Get back to us, please.