Check the outside of the case neck for a sharp/square edge, especially if it was a heavy crimp originally or if you trimmed the case. (Fired once in your rifle or someone elses? Collet dies or FL?). I've seen em drag or feed bad due to bad/no chamfer on this outside shoulder and heavy crimps on the original load make it rougher. Resizing smooths it, but not enough. My first thoughts anyway... Sometimes there's a small ring of carbon in the chamber and if your round is in spec , it might still be deeper than the ring of carbon in the chamber due to shorter brass being normal, the sharp edge makes this even more apparent. Cap dog may have a point there too, color the shoulder of a round with black marker, chamber and rotate and look for scuffs.