My shotgun has been malfunctioning. The bolt will lock to the rear even when there are shells still in the mag tube. Its almost like whatever is responsible for pulling/letting the shells out of the mag tube is not working so the gun thinks its empty and locks to the rear. Trying to correct malfunction is difficult. pushing bolt release doesn't help since it always seems like a shell is partially in the way of the lifter. I usually have to pull back on charging handle as hard as possible or multiple times, reach in through ejection port to free up a shell just to get the bolt to release and then shoot some more. Is there anything obvious is should look for or be able to see?