Hi Papaya! When it comes to high tuck jumps, the mastery comes from the push off. It is also a proven fact tuck jumps (or any explosive moevement) come more naturally to people with a higher ration of fast twitch muscle fiber(as opposed to those with a higher ratio of slow twitch muscle fibers in which they are better at endurance activities).
So if you are really struggling with it chances are your body makeup has more slow twitch fiber. But if you feel that you can get there and its just a matter of figuring out what to focus on, focus on the take off....do a slight squat and then power up vertically while quickly pulling your knees up under you.
If you feel you have more of a slow twitch fiber make up, then you can still make the effort to explode up as high as you can but instead of pulling your knees all the way up under you, shoot to just jump up and bend your heels up behind you 90 degrees.
Good Luck!