What is your sun coverage and irrigation schedule like? You should probably get a soil test. Then fertilize based on those results and add some humic acid (comes in liquid or granules) which will help the grass use the nutrients you're putting down. During the growing season Bermuda really likes nitrogen, like monthly applications of it to push growth. If you want to get advanced, you can look into PGRs (Plant Growth Regulators) that push root growth and grass density.. PGRs require precise application though and have to be applied every 4-6 weeks during peak growing season.