As
Bobbyraz49 said, the Major League teams offer you more options to improve your players (trades, etc.).
I haven't played as a minor league team, but I wonder if you can also sign Free Agents (under the League menu and/or along the right side menu look for Transactions). You can cut some of your worst players, at least in terms of not having potential, and perhaps sign some better ones to replace them. (as RchW said, those red arrows are candidates to be replaced)
Good coaches should help (especially with Development or Mechanics, as far as I can guess), but it will take months normally for any significant growth as they have to learn, also develop new habits. Defense should improve as they gain experience playing a particular position.