It been about 2-3 yrs and Im ready to play seriously again. Ive played off and on for the past yr but nothing to serious, just some 3-3, but I long for the full court game. Any suggestions on exercises.

I am 6"1 and have a wingspan of 6" I can easily grab the rim with two hands off the spot and palm the ball but when i try to dunk, the ball always hits the back of the rim and comes out. Am i not jumping high enough or is it my technique. please could somebody give me some tips. Thanks.