no clue how to fix it but I have noticed some little glitches on mine where I can click on a face, or vertice and they don't highlight until I move the screen a little or click on another face or vertice. Maybe thats your problem? Maybe you're not clicking on the dot in the middle? Try using, Edit>Paint Selection Tool, for highlighting your faces, I just recently discovererd this and I've saved time, and frustration using it to select faces. If these suggestions don't work than hopefully someone else has a better suggestion for you.