you need to manage your gameshape. for example, if you have a player whose salary is over 100k but only has awful gameshape, he may still perform like a 20k salary player. thats just an exaggeration but gameshape and enthusiasm is are things you have to learn to manage in lower league before you can promote to higher levels. try to keep your players' minutes below 70 min a week will either improve/maintain their gameshape.
other than that, train young players while you have the leisure to do so in lower level, especially for out of position training. don't hesitate to ask any questions guys! I will try my best to help!