I wonder if the problem guns screws are bottoming out? The screw will never tighten if it does.
I'm testing mine to 1000 rounds at 50 in./lbs. or I'd do it.
Could someone measure from the shoulder of the cap to the end of the thread and compare it to the shoulder of the trigger guard to the bottom of the threaded hole?
If that's the problem, all that would be needed is to file or grind a little off the end of the screw. S&W could send out new cap screws.