It looks like you have a new striker, so that should eliminate it as a source of your problem. I have a Shield 9, a Shield 45, and an M&P 40c, with nearly 30,000 rounds combined, and their strikers have proven to be extremely reliable. Since they didn't replace the recoil spring assembly, it might be the culprit. It could have weakened (if your Shield has lots of rounds through it), or it might be defective. In such a case, it may not always return the slide fully to battery, leaving the firing pin too far away to make contact with the cartridge. So ask S&W to send you a new recoil spring assembly. Then there's the ammo. Some brands have occasional "hard primers" that don't always fire (or even dent) when struck. In my experience, Sellier & Bellot ammo seems to have more of these than other brands, but there may be others as well. I'd try some different ammo, just to check.

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