I get the percentage thing and the more upgrade the less chance of it working. But when you fail you lose a star previously gained aswell as losing the medals in you inventory? that sucks when you fail over and over and had to not only get more medals but upgrade all over again just to get to where you were, and the chance thing comes into play again repeating the cycle. This is what happened to me anyway. I went on a spending spree and used up all my gems just to get enough medals to get get to where I was before and still failed to get there. Am I that unlucky???
They should make it you just lose all 3 medals and not the stars on your character to be somewhat fair and avoid people putting their fist through the screen