It could be a headspace problem caused by dried crud in the chamber, preventing the case from going all the way into the chamber. If it is, you can soak the chamber with brake cleaner and scrub with a 38-40 caliber bronze brush. You can also take an empty case, flair the case mouth, soak the chamber, and tap the case in. The flared case mouth will be tight and help cut the crud out. I have used this method when a .45 Colt has been shooting 45 Schofield black powder loads and foul the chamber.