I had taken private lessons with two coaches, both USPTA Level 1 Certified and former Div I players. They both helped me improve my serve but yet they did not catch a major flaw in my service motion. After a year I took lessons with a third coach who caught the flaw and he says it will take time, up to a year, to fully correct it. I am disappointed that the other well qualified, and well paid, coaches did not catch this problem. ..And I had specifically asked them if there was any flaw in my service form because I did not want to develop any improper habits at the outset.