There are plenty of folks here with more experience but here is my two cents:
About BB's ... I never used them HOWEVER, I got me some black pony beads that seem to be made of the same stuff. THEY HELP when getting borderline coverage.
Humidity...THIS IS IT. Humidity is the absolute most important factor in shake style powder coating.
I live in Wisconsin. In the summer it is commonly 100% humidity for weeks on end. In the Winter when it is like -25° F, the humidity is pretty much zero. I started with HF powder. Summer took two to three coats just to get a thin layer. Winter the bullets looked like bees full of pollen. All puffy and thick coat of powder. You would think that these were two completely different systems. But ...just the humidity. I have tried to get humidity down in the summer with just slight results. Doing it in an A/C place helps.
I put desiccant packs in my powder. Don't know how much it helps. Preheating the bullets a bit before swirling sometimes helps a bit also.
It takes experimentation. Good thing about bullets is it is easy to melt em and try again.