do you have the latest version ?
If not, update to it...
If yes, open your "Tennis.ini" (or the .ini of your custom profile), and then look these lines after [WT_Training] :
PotClub = 100
PotJunior = 97.5
PotPro = 95
PotMaster = 92.5
PotIncredible = 90
This is the max potentials for your main player.