I remember a "Peanuts" cartoon once where you see Charlie Brown holding an crack and each pay for the first 4 he said the same thing. Rats! over and over, then on the 5th pane, he said: "No matter how hard you try you just can't bend a cracker!" Well on the single squeeze dies, the dies are being forced to move the metal around on the planchet, that it forces them to crack. So they tend to break often on the head to the rim, Memorials crack on 1-4 corners of the building and the shields on the shield because the metal is forming so much, it is like bending a cracker. It eventually gives out. When the fields are polished down, it is even easier to see die crack happening. So it is a design issue. On the multi squeeze dies this also happened, but less often. Why Because the profile was higher and the polishing did reduce the field as quick and left the die stronger. So that is why we see so many dies breaking down quicker on the new dies. Lower profiles, and the die polishing weakens the die quicker. So they just continue to use them until they give out, then retire them.