HighUintas, Nice to see you posting again. I seems That this powder making has an infinite learning curve, or at least one that will go on for a long time if you let it. That kiln with PID is certainly a useful tool to making a very consistent charcoal time after time and its basically a set it and forget it type deal which I appreciate. The brown charcoal in my tests was the first marked improvement for my powder.
The second detail, which was much less obvious and frustrating to figure out, was indeed the density. I was getting different results from the same batch of milled powder until we figured it out. The system I use now gives me very accurate and consistent density and no special equipment is required, you just need a scale and do some basic math to calculate final volume. I like my powder about 1.6 g/cc or so, I find that it gave me the best balance of power and fouling to grain toughness.
And last but certainly not least, like VP mentioned a lot, was milling, optimal ball mill speed and length of milling time is very crucial and was my latest marked improvement in velocity. Its not as easy to measure but a properly milled powder should burn clean with a flash or puff.
As for your flow problem, I am curious about how much moisture you add to your milled powder before pressing, this seemed to occur when I used to add about 10 times the required moisture to the milled powder...moisture should be extremely slight, maybe 2-5 water sprays per pound of milled powder. More like 2 in humid climate. Finished pucks should be a medium grey, not dark.
I will experiment again with reducing the charcoal content and increasing the KNO3 in my next batches now that the other variables are controlled. Logically, it should be more efficient and produce less carbon but time will tell.