New model gun, new unproven round, new weapon needing a good break in. You have a lot of things working against you right now. I don't think it's a spring problem. Maybe the GAP burns a little dirty? Might have to work at keeping the chamber real clean. My guess is that you have yourself a new pistol that needs breaking in. Don't change anything yet. Try blowing at least 500 rounds through it before you change it. After another 100 rounds it might not miss another lick.
I can only think of two things that could hold the slide back. Empty mag and slide switch. Watch your thumb when firing that it doesn't ride to high or the recoil will push your thumb against the switch and lock the slide open. People that are used to firing 1911 have that problem when transitioning into the XD.
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