As a tennis professional for over fifty years, sometimes you must tell your student to "go with what you got!!" What does that mean? At a certain point a student's abilities will reach a static point. A small correction might not show any positive effect on the outcome of the match.
 
As a young pro I was always trying to improve all shots. It certainly works better when a student is young and athletic. As I teach older students, I try to teach them to use what they have. I teach court position, how to run certain plays and how to analyze their opponent's game and tactics.
 
I teach them to quit worrying about their individual game and only focus on how to beat their opponents. If something that normally works quits working, just forget about it and change your tactics. The match is no time to work on your strokes. Quit worrying about perfect shots.
 
Focus on how to win the point, the game, the set and then the match. Most players have enough game to be very competitive. When it gets down to it -- "Go with what you got."
 
You will have more fun and probably win a lot more.
 
Good luck on the courts!