There could be a number of reasons. First check to see if your water is moistening the soil at root level. Check for any insect pests that may be feeding on the foliage or on the roots. Are you in a warm summer climate? Somettimes it helps to aerate the roots by using a plugger that will take small chunks out of the grass and roots. Also read up on "growing St. Augustine grass". If you have further questions or comments e-mail me- George.