It is unfair that you lose all the boosters you built up if you cannot succesfully complete a challenge level. The mere definition of "challenge" means that you have a choice to accept or not, but not in this game. Either give an option to skip a challenge level, or allow gamers to compete but NOT lose all boosters built up. Challenge levels are normally twice as difficult as "super-hard" levels. I changed my rating from 4 stars to 2 stars due to this.