First off, I am new to AR rifles. I bought the M&P 15 sport 5.56 and haven't even fired it yet. The buffer retention pin fell out and logged itself between the hammer and the lower frame. I removed the hammer to get the pin out and reinstalled both. After a few dry cycles the pin and spring fell out again and jammed up. Any idea why this is happening. I plan on taking it back to to store under the protection plan I purchased to have it fixed but was curious since so many people rate it as a good, reliable rifle. I don't want to fire it before they fix it and risk blowing it up. The only thing I have done otherwise was adding a single point sling plate.