Dented shoulders

I ran 20 rounds through the Redding full length sizing die again this last week. I first cleaned the die then I used imperial die wax and a nylon coated brush for the inside case necks. Paid very close attention to my operating speed and amount of lubrication. I stopped at 10 and cleaned the die again. I made it through the first 10 cases without a single shoulder dent. When running the second batch of 10 I dented a case on number 10. It never dents on the case body or shoulder junction. Just the face of the shoulder. I will attach a pic.
Maybe an added prep only but clean the necks with 0000 steel wool before seizing. Clean and shiny on metal is always best especially on case necks.