you're probably a kid who doesn't understand statistics. so just trust us when we say it's normal, it's just random.
THERE'S NO GUARANTEE. if there was a guarantee, everyone would be playing lottery because everyone would win. but most of people don't win, and that's how the lottery companies earn money, they earn money on the people that don't win. if everyone won, the company would be out of business and no one would make lottery games because they aren't profitable. even if you played lottery your whole life, you probably won't get anything (you might, but it's not guaranteed, it's just random).
that's like saying you want a SU to drop out of *every* monster because there's like a 0.05% chance for it to happen. ridiculous.
there's nothing to fix. what has to be fixed is your understanding of statistics and your attitude.
the only thing I can see going wrong is if you're cheating and trying it on the same item all the time, in which case it won't work. If I remember correctly, if it fails on the item, it will always fail, since the chance is rolled with the item, not when you cube.