Plastic-bodied shotshells sometimes leave plastic residue in the the chambers, and, once it gets heated up, causes shells to stick in the chamber. Make sure your chamber is squeaky clean, and use a solvent that specifically states it will clean plastic residue out of shotgun chambers. I would recommend using a nylon bristle brush while cleaning your chamber, as well. The residue is hard to see, so you are really going to have to work at it.