You can try two things:

1. Before filling of bunker dry powder at 100-120°C and then filled it with 50-60°C. You can also use heating sleeve on feeder

2. powder washing with water  on a sieve 25 microns or 36 microns (dust is washed away); drying at 100-120°C and fill it warm into the bunker.

Thanks for your answer, Vadim. The moisture does not  seem to be the problem.

It may also be due to fine grain, in this case help the washing.

powder washing with water on a sieve 25 microns or 36 microns (dust is washed away); drying at 100-120°C and fill it warm into the bunker.

Please, check the gas carrier pressure and rotor in hopper, it could be stuitable condition  non-enough for feed. Normally, the morphology of powder was able to move in hopper.