No shotgun expert by any means,I don't have hands on experience with the Model 11 but I have worked on a couple Browning Auto's and a Savage, I don't recall which model, that are in all respect the same as the Remington. One of the first things I would look at is the cleaning. Since this shotgun is semi auto it gets a lot of crud built up in the action which could be gumming up the parts that feed the shells , even a dirty magazine tube can cause feed issues. Also the friction ring not being set for the shells you use could cause problems. If not then look to the action bar and the shell stop for crud buildup, burrs or dings that might be hanging the action up long enough to let the shells run out.
Try cycling rounds,preferably inactive , through by hand, with the magazine fully loaded and see if you can spot the problem.