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. 

From my experience I would say the grain size is ok. I would also recommend checking the feeder stirrer inside the feeder. These fail quite often due to the construction of the cable connectors at the feeder top. Dismount der Cable adapter and check, if the wires are still correctly brazed to the contacts. We have a Metco twin 120 feeder and this problem occurs twice a year.

If the feeder is fine and your problem remains, I would try to add a vibratory unit to the feeder. I have seen this already integrated in a feeder unit from THERMICO which is specially built for fine poweders. The vibrator unit provides a much better and more constant powder flow. Therefore we made one ourselves, attaching it with a lashing strap at the cylinder unit. It is just a simple eccentric rotating weight. I used it for Al2O3 Powder of size 15-45µm and it worked out very well.

I think this will help.

Best regards