As in real life players can improve or worsen and training has a big impact on it.
Remember to plan every week the training for your first and youth teams and see what aspects are better to train for your playstile. Depending on their form, morale, age and potential they will progress more or less. Once the training is done you can check the impact of the drills on the results tab.
If planning the training every week is too time consuming, you can always use Ucoins to delegete the training functions to the coach.