i wouldnt say that sasuke isnt getting stronger under kakashi...he has gotten his strength from being under kakashi...he has one pupil in each eye to go to have mastered the sharingan...he awakened his sharingan against haku at the start of the series...he has come pretty far in one year.
yes itachi had mastered it at 8 or whatever but it took him a couple more years to obtain mangekyou sharingan...
lets see how long it takes sasuke?