To begin with, back off your sizing die so there is an 1/8-1/4" (or more) off the shell holder; then, lube your case and FL size it.
Check and see if the empty case chambers. IF IT DOES, bring the your sizing die down a little more, say about an 1/8", then repeat process.
What I think is happening is that your sizing die is too low and pushing down the shoulder of your case,
which causes a bulge at the shoulder making it too wide.
Personally, I full length size all my brass (except a couple of pistol cartridges) for best accuracy, then trim to insure case mouth squareness.
Some cases stretch more than others, which I think is related to the shoulder angle. For instance, a 280 AI case stretches very little if any,
while the the 280 Rem stretches each time the case is fired; and the 7x57 stretches a lot more than the 280 Rem.