Common problems include fungus diseases, incorrect pH, poor fertilizing and lawn mowing practices. Your local county extension agent should be able to help you identify fungus, correct pH and suggest better fertilizing. If you're bagging your clippings, stop it. You may be removing topsoil and organic matter. Set your mower to a greater cutting height. Short leaf blades will eventually starve the grass. These are but a few possible factors to consider, but should be a good place for you to start.