I had a new Mk III that did the exact same thing and here's one possibility.
The empty is being knocked off the extractor by the rear feed lips of the mags. Replacing the factory extractor with a Volquartsen will usually solve this as will rounding off the front of the rear feed lips on the mags. I did this with all 3 of my factory mags and no more problems. Go here for the best info on Ruger .22s...
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