Assuming the entire gun has been totally field stripped (including the recoil/action spring, follower and tube), cleaned and lightly lubed in the appropriate places...the first thing to do is make sure you are inserting the shells all the way into the mag, past the shell stop. The Mossberg is like the Beretta and Benelli semi-autos...you seem to have to push shells an extra 1/8" into the mag tube compared to Browning/Winchester and Remington offerings. If that's not the problem then run at least 200rds of heavy loads thru it to break it in. If that doesn't work...send it back to Mossberg for warranty or replacement.