problems with training

i want to train my players naturally but for some reason i cant improve the defensive attributes of a player (a defender) when a skill point is achieved, it allows me to improve, physic+mental, attack and special ability. why cant i improve his defensive attributes and how can i fix this? (i know it isnt maximum because i have other players with higher defensive attributes)
thanks

You have reached the maximum difference of ex amounts of skill points between the highest and the lowest. If your raise your lowest "attack for you I guess" you will be able to spend more points in defense.

You cannot have one area that is more than 20 higher than any other area.