By chance are you using Federal brass? A buddy of mine started blowing primers and it was all Federal brass. From what I have read they have soft primer pockets and the internal dimensions are somewhat different then many of the other brands of brass. The difference with the Federal increases cumbustion pressures and can cause blown primers among other problems. I only re-use my federal brass 1 or 2 times then I scrap it for these very reasons.
As far as I know blown primers are a sign of exteremly high pressure or soft primer pockets



























