I think remarkable has it right. If it were a spring rate issue, then it generally closes on an empty bolt and never releases next shell. I would try a different brand of shell before you try to have it serviced. Sometimes a tiny manufacturing product (metal shavings usually, or not as polished as it should be) will cure themselves out with a diet of heavier loads. If you find a load it does like, most likely you'll be able to come back to these later after it is broken in. If nothing works, then they should be able to adjust that. If memory serves, that only takes the tiniest of change to adjust so if you are a DIY'er, don't grab the Dremel.