The rest of the four sides should be as small as possible. In addition, measure the balance of upper and lower dies (upper and lower dies after final assembly). The most important thing is whether the length of each thimble corresponds to each other. In my experience, if the length deviation >; 0.0 1mm will cause the phenomenon you mentioned. If the length of thimble cannot be guaranteed, it is necessary to thicken the thickness of blank holder ring, increase the R of all corners and sides at the same time, and then increase the blank holder force. I hope this little suggestion can help you.