Hard to say, but there are three things that comes to my mind, if you fall backwards BEFORE you are on the new tack:
Maybe you fly the kite overhead too early ?
You have to lean against the lines, while you turn the board up through the wind.
Maybe you only push with your rear foot ?
You have to pull with your front foot - very important as the big fins wont allow you to turn the board so your rear foot can not push it around, but the front foot can easily pull it around.
Maybe you are too slow ?
You have to make the upwind turn and kite direction change and jump around quite fast, and in one fluid motion.
Just some basic ideas that might be worth thinking about - hope this can help you ?
If you fall backwards AFTER the tack, it is most likely because you fly the kite around TOO LATE, so it wont have speed and tight lines
Also, when underpowered, the tack is MUCH more difficult to make, as you wont be "weightless" the same way, and your timing has to be perfect - so practice when quite powered first, as much easier