Could be just that when you adjust your "bullet-seating" die down, you seated it to tight... Turn the bullet seating die down on an empty casing (that you have sized and then trimmed to length) until the die touches the shoulder of the case, then back die off 1/2 turn, lock down lock-nut, and readjust seater.. Sometimes when the die is set too close it rolls the shoulder of the case.... ALSO check the OAL of your rifle and seat bullet back a bit, say maybe .010, or so.
