Why can't I dunk I need help!?

I've been working hard I don't know what I'm missing I'm 6'3 and I can grab rim and hang on it with 2 hands off vert wtf Why can't I bring one down?When I try I get up but it bounces out.Maybe it's my control of the ball?I don't palm it when I go up but maybe I should cradle it in my hand?HELLP PLEASE VERY EMBARRACING