I know this is going to sound strange but it happened to me so I know it is a possibility. I was shooting 777 with some 777primers. Beyond a 100 yards it started to string vertically. I had 6 shots in a vertical line. I switched back to Federal primers and the vertical stringing stopped. I don't have an explanation but with muzzle loaders I've learned to never rule anything out even if it sounds ridiculous. Try a different primer and try to seat the bullet the same.. Just a thought