The S&W revolvers lock at the front and back of the ejector rod. The lock pin enters the receiver hole and in the front a spring loaded clip enters the tube that is the ejector rod. When you try to release the cylinder the thumb piece pushes the lock pin from the hole in the breech face and the other end of the lock pin pushes the front locking clip from the front of the ejector rod tube...Most common cause is the ejector rod unscrewing itself from the front of the yoke assembly, remember this is a left hand thread! Worn or blocked thumbpiece can cause sticky release as can a worn or broken locking pin.. It must be long enough to push the front locking clip clear of the ejector rod tube. Check the thumb piece goes flush to the breechface when activated (watch with cylinder open). Check that the locking pin front pushes locking clip clear. The repair may require a new locking pin but if the pin is just a tiny bit short I have stretched them..with a light hammer.. S&W has changed this piece's dimensions (length of pin in front of stop collar) but I don't know if about yours. If this doesn't help let us know!
