The loading cycle of a tubular magazine shotgun is a delicate balancing act. There are devices Winchester calls shell cutoffs that move in and out controlling the release of a cartidge, yet catching the next one preventing problems. It is kind of hard to explain, but suffice it to say they can wear out causing the described problem. The more likely cause is fouling such as unburned powder residue interfering with the proper movement of these devices. Remove the trigger group, barrel, mag spring and follower (bolt if you can) and flush the receiver with gun scrubber to get the gunk out. A good cleaning solves many of these problems.