There are several products and ways to make your hair look fuller. One of the easiest that I came across was at http://www.forhair.com/Articles/Hair_growth_vitamins_and_minerals.htm which is to dry your hair upside down with a hair dryer then flip it back over when you are done. Drying the roots first seems to help a lot.
You can find many other tips as well. If you cannot find a proper solution for your problem, I would recommend contacting a professional doctor.