Static. Moisture in the powder. Oil vapor in the air that has condensed on the tumbler's surfaces. All possible. Clean the trickler with a strong detergent and allow it to air dry. Check the powder for moisture content by pouring some of it on a clean, dry sheet of paper and tilt the paper to see if it slides around without clinging/clumping.
I got so disgusted with commercial tricklers I made my own. Used a 4" section of 1 1/4" brass drain pipe, a sink "tail piece" for the body (shut up now, that's what it's called!
). I filled the bottom couple of inches with lead. Used a 5" x 1/4" diameter section of auto antenna type tubing and a Radio Shack plastic knob for the feed tube. Epoxied the body on a 3" square of 1/2" steel for a base. It's heavy enough to be stable and works quite smoothly, much unlike any light weight tricklers with aluminum tubing. It's easy to clean too.