Is the brass you are using necked up 30/06 cases or factory 35 Whelen.
If 30/06 brass the donut formed in the neck might be your problem. When you neck up part of the upper part of the shoulder on the case being necked up forms the bottom or lower part of the neck on the necked up case and the brass in the shoulder is often quite a bit thicker than brass in the neck.
A quick way to test this is seat a bullet well out so the bullet is not inside the donut and see how that chambers.
