First, in spite of the nearly universal mis-use of the word "timing" in this circumstance this is not a timing issue. It is, possibly, a "carry-up" issue. Timing refers to the relationship of the charge holes to the barrel bore when "in battery". Carry-up is how the cylinder is advanced relative to the cylinder stop notches when the gun is cocked.
You need to check carry-up correctly, and cocking the gun slowly isn't it. While applying very light pressure to the side of the cylinder with the opposite hand to slightly retard cylinder rotation pull the hammer to full cock, hold the hammer to prevent it falling, and pull the trigger completely. If, while being retarded, the cylinder stop correctly falls into the stop notches then everything is good. If it doesn't drop until you manually turn the cylinder slightly then the gun doesn't carry-up. The only way to correct this is to fit a new hand. A new standard hand may be adequate as there is slight tolerance in thickness. Otherwise an oversize hand is needed.
Note: Be sure you have cases in the gun when you check it, this can make the difference between full carry-up and not.
In addition to an oversize hand you will need a "Hand window" file, in case the hand window is not wide enough for the oversize hand. A small "barrette" file, to file the ratchet lugs as needed. And, finally, a thorough understanding of the proper relationship between the ratchet and hand and how the hand acts on the ratchet, the manual dexterity and patience to do a workmanlike job, and the skill that is necessary. Even with the correct tools a single incorrect file stroke can ruin a ratchet beyond repair and totally defeat the repair effort.
In other words, find someone who already knows how to fit a hand and pay them to do it. This is assuming that a correct check for carry-up reveals that a new hand is needed!
