Have you tried renting an aerator? After aerating, apply 1/2" - 1" of good quality compost. You will need to dump little piles of it and spread it with a rake out from there. The results will impress you.
What is your watering schedule? You should water deep. From what I understand, the roots will grow longer the deeper you water. Also, mowing height is important. The higher your grass is allowed to grow, the deeper the roots will grow. I like Bermuda grass about 2" -3" tall. Remember to increase mower height for slopes to avoid scalping.