Is it possible that someone opened it up, and in the process of removing the trigger, pulled the hand too far back to clear the frame, thereby causing the torsion spring that keeps forward pressure on the hand to slip from behind the stud in the trigger, resulting in the hand having no forward spring tension to maintain engagement with the ratchet when the trigger is pulled?
This can be checked without taking the revolver apart by swinging out the cylinder and putting rearward pressure on the hand through its window in the frame and after pushing it back just a little (with the revolver pointing straight up), checking to see if the hand springs forward under spring pressure. If there is no spring pressure forcing the hand forward, then this is the problem.
If this is not the problem, then the revolver should go back to S&W.