Try a different mag. I would bet if you look it's the follower hanging up on the edge of the mag body. A different slide stop can work too. I have had this with colts it is a combo of both part working together. Usually the mag that comes with the gun doesn't do this and it's also these two parts not playing nice that keeps the slide from locking back on some empty mags. Better to have your problem as it won't effect a quick reload.

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