training is just a way to shape a player to your needs
if you want them to develop fast you need to secure a few things for them
training facilities, tutors and playing time
their personality plays a huge role too but it can be fixed by tutoring
just make sure they play a lot in either your youth team or first team, if they can't get enough playing time then just loan them to a club that will play them
i usually sign players at 16 so i can tutor them 2 years and shape to my wishes, when they hit 18 i loan htem for 2 or 3 years untill they reach their full potential or are ready to play for the first team.
If i signed a player who can joing my squad at age of 18 i make sure he has the fitting personality (ambitious is proven by 2 experiments i read as the most important attribute/personality for player development, 3:1 ratio amition to profesionalism) and then i play them 1 year or 1.5 year depending when they sign, in my u 18/u19 squad and make sure to tutor them twice
