Sir, welcome to the forum. Some possibilities, one is the hold open is worn, second is the ammo isn't powerful enough to kick the bolt back far enough to lock, that can be either the ammo or the mainspring being too strong for that specific ammo. Recommend trying several types of ammo, sometimes a heavier bullet also will help. Its not an uncommon problem. Takes a little shooting with different loads to get the right combination for your gun. Spring replacement kits are relatively inexpensive. Also careful cleaning of the slide and rails and greasing them before shooting also can help.
rk
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