Help Skill_Trees

hi there, i want to ask something about skill_trees, skill trees is where i can add/remove skills from characters right? i tried to add some skills to characters , for example i tried to give , hawk eye and ultimate defense to fortun seeker and its got only ultimate defense, i tried to give acumen  and celestial shield to some characters and they got only celestial, so question is why? why i cant give acumen when i give them celestial shield? can any1 explain how its works?