As the above posters have advised, too big a reduction in one step, too much lube on the case or an accumulation in the die. If you can reduce to .35 caliber prior to final forming may help. take your die apart and clean thoroughly especially the neck area. I don't know what lube you are using but when I experienced oil dents on the shoulders I switched to a sizing wax. I use a product called Imperial Sizing Die Wax, it is applied with the fingers. Wipe your fingers across the wax and apply to the neck shoulder and first third of the case. Just a light application is all that is needed, I lube 3-4 cases before wiping my fingers in the wax again. I lube the inside of the necks with Motor Mica which is a white graphite, Forster makes a brush set-up to apply this.
