Does the action work fine if you attempt to cycle it manually? Does it have any issues ejecting the fire shell from the chamber?
The first thing I would do is make sure it is 100% clean...that includes the action, bolt and most importantly, gas ports. Secondly, I would recommend that you pull out everything inside your magazine tube which includes the shell follower, spring and plug. You will see that mossberg uses a 1/4" wooden dowel as a magazine plug. I would recommend replacing this with a plastic plug. I own a 935 and had a similar issue to what you are describing. It turns out that a very small piece of the wooden plug split and broke off while shooting 3.5" shells and it become lodged between the shell follower and the magazine tube. This prevented the gun from feeding shells from the mag tube. It was a 2 minute quick fix in the duck blind, but be very careful when you pull the magazine spring retaining cap off because the spring is under load and will send the cap flying once its loose.
Post your results and or additional info so that we can further understand what is happening
