I don't see anything in the screenshot that would cause the problem. In the screenshot the layer thumbnail is selected; the mask is not selected, but that may just be a coincidense. Are you by any chance using a brush that is less than 100% opacity? If so, try selecting one of the hard edge brushes in the General Brushes folder.
Now you have shown us you have changes your brush to 100% opacity. Show us now painting on a mask with that brush. Alt+Click on the layer mask Paint and capture the screen so we can see Painting is 50% Gray not black.
That look like you are using some sort of sampled brush tip. Try a hard round tip.