This seems to be a common complaint that pops up from time to time on forums and in Community Help. Possible solutions?
Flow, Density, or simply the effect might be at a low value, meaning it’s not really visible. Increase them to solve.
Another possibility lies with the mask overlay. Pressing O will show the mask, so you can paint where you want the brushing to show. Shift O lets you cycle through the colours, which include white. If the area you’re painting is a similar tone to the white, it effectively makes the brush invisible. Not so good! The solution is to use a different colour! And yes, I know, because I got caught with it myself!