Screw a threading tap into the brass (not too big) and pull the brass out. Have also used a larger brass brush on a cleaning rod, in your case go with a .270-30 cal and give that a try. Insert it from the chamber end and into the neck a bit, then try and pull it back out. Either you are pushing your shoulder back too far when your resizing (too much headspace) or have just reloaded that brass to the end of it's life.