i think it's lack of touch. find a workout for big men online or on YouTube that will work on your inside game, like layups, bank shots, hook shots, mini fadeaways, etc. Start close. Once you can make like 10 in a row, move back a step.
if you really want help, post a vid of your jumper and you dribbling, we could help better if we get a better idea
if you really want help, post a vid of your jumper and you dribbling, we could help better if we get a better idea