I've figured out how to slash, pick and roll, etc to get a nice drive to the hoop but he will layup 90% of the time. Even though my dunk is 95 and my vert is 90 and even my standing dunk is 85. (I'm 6'7").
I have all available slots filled with a variety of dunk packages, but he just will not dunk.
Yet the AI is constantly throwing down monster dunks, alley-oops, and drives that start at the 3-point line ending in a dunk.
Check you stamina when you get to the rim. If you're tired, you won't dunk. The dunk back doesn't matter because if you're going to the rim, you should dunk if you're holding RT and then shoot.
Your go to back is generally the one that works most of the time for the situation. Try doing to practice and see if your guy dunks. My Players dunks all over the place with the base packages you get as an athletic Small Forward.
Definitely has to do with stamina, if you don't have enough, you won't dunk. Also I do believe it has to do with the specific dunks in whatever package you have. I can't seem to do any other dunk besides Jordan's free throw line one with his dunk package. I know when I had Tomahawk in an extra slot, I would throw down much more often.
For instance, the 360 between the legs has to be done as a spin dunk...Figured that out after a while.