Well, I'm far from being an authority but just off the top of my head ...
1) Bullets wedging in the magazine and not advancing. But then manually cycling wouldn't
help.
2) Weak magazine spring not advancing the next live round fast enough for the bolt to grab it.
3) Bullets not producing enough recoil for the pistol to fully cycle.
4) Not holding the pistol tight enough to allow the pistol to fully cycle.
Just ideas ... by no means gospel.
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