PW, have you tried changing the colour of the brush or the background colour?  Some brushes are designed to be used 'white on black' (or any other light colour on a darker surface) or 'black on white' (again, any other dark colour on a light surface). 

 

I made this image with purely brushes, but I had to have the background black and the brushes white for them to show up properly (except the trees, they were black brushes) - I had the same problem as you:

http://www.deviantart.com/deviation/35773408/?qo=19&q=by%3Aelfdaughter+sort%3Atime+-in%3Ascraps