One of three things happened: 

1. You recently washed your brush, and water got into the white tube. No problem, this is an easy fix. Straighten a paperclip, spread the bristles to find the white tube in the center of the brush and insert the paperclip into the tube. Move it around in a circular motion to clear the clog. 
2. Your brush needs to be washed. If the bristles get too oily or dirty, they will clump together, which makes it difficult for the powder to come into the bristles. Clean your brush and the problem should be solved. 
3. It’s time for a refill. It may look like you still have powder in your brush, but when only the bottom of the yellow “window” has powder in it, the flow will slow down. Add a refill, and the flow should return to normal.