When I first got my S & W 629 about a dozen years ago, I tried the internal lock. That's it. I unlocked it and never thought about it again until this past April.
It had been about three years since last firing the big .44, when I decided to bring it out of hibernation and dry fire it a bit. Guess what, it was stuck. I did not want to force the trigger, so I checked the ejector rod to make sure it hadn't backed out trapping the cylinder. It was okay. Then I took off the stocks to see if the strain screw had backed out. Nope. That's when I went online to the S & W forum and found a reference to the IL needing a bit of lube. I took off the side plate, lightly oiled the IL and reassembled. Worked like a charm.
The moral of the story for me (besides gun forums being a great resource): Internal locks should never be put in a gun that might be used for self-defense.