I need help with my vertical and my technique for dunking.?

I'm 6'5, and I have about a 23-24 inch vertical. When I jump straight up, My wrist gets about 2-3 inches higher than the rim but I still can't dunk. Everybody says that if you can get your wrist higher than the rim, you can dunk, but I just can't. I think it's my technique or maybe I just need to increase my vertical a bit more. I seriously need some help. Thanks