My first thought is that you're short stroking or soft-stroking the lever. It doesn't need to be abusive, but it needs to be operated 'smartly', if that makes any sense.
Then I would look for any obstruction that prevents or delays the cartridge carrier from fully bottoming out. IIRC, the next round in line should move part way back onto the carrier as soon as it is low enough.
There is a detent ball on the side of the carrier that may be sticky and hanging up.
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