I have just gotten my shipment of 404 Jeffery "basic" (unformed) brass that that I planned to reform to 404 with maximum headspace. I was told that this should be a one-pass operation.

However, when I run the brass through the sizing die, it comes out creased on one, or both sides, at the shoulder.

I have only been reloading for about a year and have never attempted anything like this.

What am I doing wrong?

GA = My guess is you're over-greasing. I've never done this particular cartridge...but my experience is if you have any lube from the shoulder up, it will "grease dent" the hell out of your case. Clean out your dies good and try lubing just the bottom half of the case and see what happens.

Pecos45 probably hit the nail on the head.Most of the trouble that I've had with creasing and buckling has been because of too much lube on the neck and shoulder area.
Check the case mouth ,the least bit of imperfection on the case mouth will sometimes cause them to buckle.If the mouth is not perfect ,trim enough to clean it up.
It is a good idea to remove the decapper rod
before forming,if the case gets stuck in the die
you can punch it out with a rod.
I have sized cases as much as .125 without buckling or creasing.Just lighten up on the lube and try it again.
WC

Problem solved!

The trouble was that I had my press mounted on a table that was way too flimsy for a reloading press (an old dining room table). I couldn't exert enough torque to reform the cases in one smooth motion without breaking it off the table. Consequently, I was reforming my brass by little short strokes to get the case into the die.

I finally moved the press to a solid work bench in the basement this afternoon. After I got it mounted, the first thing I did was run a 404 unformed case into the sizing die. To my surprise, it ran up into the die smoothly, and when I let it down, voila -- a perfectly formed 404 case. I then did this 10 more times without a failure.

I never realized the disadvantage I was at by not having my press mounted on a solid surface.