The Mane Six do all tend to find difficulty with their elements. Rarity can be greedy, Fluttershy has anger issues, Pinkie Pie is prone to depression, Applejack is prideful to the point of overstating her abilities, Rainbow Dash is somewhat narcissistic and Twilight is antisocial by nature (Magic=Friendship, remember?) But the fact they have to work at it makes their embodiment of the elements all the more powerful, rather than if they were things that always came naturally to them.

I lost it at "Magic"