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After ruling out extractor/extractor rod problems, I would look into the action. First, disassemble it completely and make sure that it is very clean and appropriately lubricated. If the revolver has had a trigger job, there are a couple of potential problems. One was addressed earlier; the mainspring strain screw needs to be tightened enough that the spring will not knuckle. If the screw is not tightened all the way it can back out. Also, if the rebound spring is too light, it will not be able to reset the trigger reliably.
You should carefully inspect the rebound slide, inside and out, and the hammer block for any burrs or divots that could make it hang up. I had a Model 48 with a very nice action that would occasionally exhibit a very similar problem to what you report. A burr in the hole where it engages the rebound slide pin would sometimes give just enough drag to keep the trigger from resetting, locking up the action.
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