Technical changes take a long time to become automatic, especially if the player has had a faulty technique for a long time. It is very difficult to erase the ingrained motor programs. It takes hours and hours of repetition, but what makes the process even more complicated is having to break down the negative mental triggers resulting from months of hesitation, fear and lack of confidence.
When a player has a weak shot, the last thing he wants to do is hit that shot under pressure, so every time he has to, he becomes anxious. Anxiety causes him to get tight, stop moving his feet, wait for the ball, fail to track the ball to contact and muscle the ball, among other things. After a while, these feelings and actions become a habit and the player no longer realizes they are there.
Any of these factors will lead to an unforced error, even with perfect technique, so it is not surprising to find players that continue to struggle with a weak shot even after the technical problem has been solved.
Therefore, it is imperative to address the mental aspect when making a technical change. An effective starting point is to make the player aware of the problem by comparing his mental approach to his best shots to his mental approach to his weakness.
Simply, ask the player to hit his favorite shot and focus on how he feels as he approaches the ball and during the swing. Ask him to focus on his tension level and emotional state, as well as on his intensity and footwork. Then, have him go through the same process as he hits his weaker shot. The player should identify differences and then work on approaching his weak shot exactly the same way he approaches his strengths. As the player progresses in this exercise, he will be able to separate technical mistakes from mental mistakes and will be well on his way to experience real improvement.