Maintaining proper chemistry...not a strength

This is my 3rd summer with a 30' AG pool and I have yet to swim for more than a few weeks because I can't seem to be able to make enough chlorine to keep the water from growing algae! I test the water from the pool place that installed the pool and have "great" levels for everything, including cyanuric acid, but there never seems to be enough chlorine! Is there a way to be sure my SWCG is actually working? Are there other problem areas that I could be overlooking that contribute to my chlorine problems?