The torsion hand spring may have a small sharp burr on the end of the arm where they are clipped off during mfg'r. The one end engages the hand pin .

That sharp (and very hard!) burr can be running against the inside of the cut of the trigger and can occasionally snag the spring in the compressed position when the trigger is pulled.
That leaves the hand with no spring action on the next trigger pull, it's just flopping back and forth and a fail to rotate the cylinder is a very good possibility (unless the gun is pointed downwards and hand falls against the ratchet).

That false pull will usually pick up the snagged spring once again and engage it and the gun seems to function fine there after.,,Untill that nasty little burr just barely catches itself into the wall of the cut inside of the trigger and hangs up again.

The trigger is casehardened, but the hard spring wire will wear through that in no time especially a sharp chisel like burr on the end from a snip-cut off operation done during it's manufacture.

Just something to check for,,may not be the problem at all.
But it takes nothing to check other than very close examination and costs nothing remedy other than a stone or diamond lap to the end of the spring arm.
Make sure the spring arm isn't bent also..