I broke a couple of them, turned out there was a mistake with my technique.
I would get into such a grove pounding the dimples into the skin, I would start to lift the skin a millisecond before i was finishing pounding, eager to move onto the next one, this would ever so slightly (were talking thousandths of an inch) misalign the dimples and my pounding would weaken and eventually break them.
I am not sure if that is what is happening with you, but try checking that out.
Also, if you put the male on the bottom, then the large skin piece could be putting lateral pressure on the die which could also misalign it.