There are two limitations to training:
- Maximum player abilities: players all have a maximum possible value for each ability. You cannot train them over this value but can still improve their abilities with gear.
- Coach abilities: a coach cannot train a player further than his own abilities.
You should look for a better "Basic" trainer to start with.
Indeed, there are several types of coaches and each type has his own set of exercises. Your team's default coach is a "Basic" coach and unlocks an exercise for each ability. The other types of coaches ("Technique", "Physique", etc.) do not improve the default exercises but unlock new ones. See "How to unlock new exercises" for more information.
Players have a hidden maximum value for each ability that we call the potential. Once the player reaches his potential for all abilities he cannot be improved by training anymore. However, you can exceed the potential with gear and building bonuses.
Although the potential is hidden, the Recruiter Kit displays the maximum potential ability on player profiles and you can reveal all the player's potential abilities by click on the "Potential" icon on a player's profile for a small vitamin cost.
It represents the ability bonus provided by player gear, buildings etc. The exact value is shown in brackets next to the total ability value.
The importance of each ability depends on your team's strategy and the player's position on the field. You can see each position's "profile" on the strategy page.
If a player does not have the right profile for his current position, certain of his abilities might be higher than the required values. In this case, the exceeding part is grayed to show that it does not improve his performance in the current position.