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Most likely you have a broken or missing ejector. The rounds in the mag assist in ejection until the last one then it just gets extracted and remains on the extractor.
Next question, what ammo are you using?
I wonder if a lower powered recoil spring would fix the issue , just weird that its only doing it on the last round.
Defective extractor, or ejector. Weak loads, dirty chamber. Wrong weight recoil spring for the load used.
Your slide cycles far enough to pick up another round, but not enough to lock the slide back on last round??? Maybe your gun is just short stroking.
Try a different mag. Replace mag spring with new one.
Do I need to mention limpwristing?