I had the same problem, and the cause was the hard (4150) steel sharp leading edge of the cylinder's rachet digging into the top of the very soft Italian hand (pawl). If you disassemble the gun and get the hand out so you can hold it in your fingers, then mount the cylinder on the frame backwards so you can see what's going on, align the chambers to the 6/12 o'clock position and hold the hand in the start position of the lower rachet tooth, (on the left side against the cylinder pin as you see it now) you may find as I did that the ratchet corner is picking up the hand at it's very edge, and either dropping over the edge to the inside (right), or as mine did, actually dug a small groove in the top right edge of the pawl's top surface, which is enough to lock it up.
I took an Arkansas stone and broke those 6 rachet corners a bit to make it less of a cutting tool. Then I lowered the top surface of the hand to the bottom of the groove made by the rachet, and checking with the reversed cylinder and holding the hand in its pickup position, got it so the rachet picks up enough outboard (toward the center of the hand's top surface) so it would slide sideways rather than dig in. It doesn't take much, but it also didn't take much to cause the problem.You can also check the relationship of the hand and rachet at the top of the sroke, and you will see that the side corner of the pawl is against the side of the upper rachet tooth and might need a very small chamfer to set the cylinder aligned straight up. It's a tricky bit of geometry, and it's easy to remove too much metal.
The main cause is the very soft steel Uberti and Pietta are using to make internal parts. A new hand made from better, harder steel is the real answer.
