It takes a specific amount of stars to advance to the next group of levels. If you are low on stars, go back to old levels and play them again, starting with older ones first, in order to earn more stars. The higher the score that you get, the more stars you earn, so try to finish with as many moves as possible, so that they can turn into potions and blow up at the end of the level. If you get potions appearing on the level, but you can win without tapping it, then do moves that don't involve tapping the potions. Leave them there and after you collect all of the soaps that you need, the potions that are sitting on the board will blow up.