It's a timing issue... the hammer or trigger could have been replaced at some point, and fitting hasn't been properly done.
The trigger, when you fire double-action, has to engage a spur on the hammer to move it back... this in turn works everything else that rotates the cylinder.
If the trigger is coming back, and hammer is staying forward... this is what's most likely happening--the trigger is not catching the hammer engagement.
I broke a firing pin on an old model 10 Smith once and I put a gun-show used parts vendor hammer in it (hammer had the firing pin riveted in it)... then because of ill-fit, my revolver did what yours is doing.
I think I ended up getting another trigger and making it work that way.
Just hang on to the revolver until you get it fixed. Lots of folks would trade it off, and someone's life might end up depending on that gun at some point.
Should be an easy fix, have your gunsmith look at the hammer and trigger fitting once again...
It will require a new part, most likely, as the ledge engagements in question are too short, rather than too long.