There can be two main reasons. First, your tennis player cannot improve beyond the maximum Overall Score imposed by your staff members or by your player potential (see What does a staff member of higher level bring me?); make sure to check this information for every staff member participating to a training. Second, the more your player is advanced in his career, the harder it is for him to improve his characteristics permanently. Here also, start by hiring staff members of better quality, who will improve the likelihood to consolidate your temporary points into permanent points. If it is not enough, it is maybe time to consider changing your tennis player and choosing one with more potential, who will be able to reach a higher Overall Score more easily.