unless you want to train players for a few seasons before selling them youre waisting your time training just any players, buy players of at least 15 - 20 stars and make sure they have good training speed and recovery %, a good player to buy and train will be like this:   [6-7] Jared 15*  19years old 3.23/h  187% 
If their train % is under 100% i'd just sell or dismiss them and get something a bit more decent to work with.