In your case, it was obviously the strain screw. In the past, I had some misfires due to moonclips that became bent from loading/unloading.

I switched to the plastic Rimz moonclips, and I never had any problems again. I use them on my 625 and 610. They are much easier to unload, and they will flex back to their original specs.

Ez Moon Clips

P.S. One of the reasons I always switch to traditional S&W two-piece grips is that the screw is exposed and easily adjusted. They do not seem to ever "back out" on their own.