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.
Special potions
When you make special combinations, such as four or five in a row, you will earn various special potions. These will have a special effect when you tap them such as knocking off one line, or a 3x3 box of soaps. Mix two purple potions together and you'll get a rainbow potion, which you can tap for even bigger effects, such as knocking off 3 lines or blowing up a 5x5 area of soaps.
Various Gameplay Tips
Tip 1: Match four pieces of soap to create a bottle with a special purple liquid. It will automatically match soap in a particular pattern. Place two bottles next to each other to make a rainbow bottle that has more power. The explosion pattern of the bottle can be determined by its markings. A "+" indicates that it will explode in a "plus" pattern, while a horizontal or vertical line indicates explosions in those directions.
Tip 2: Many levels require you to match a specific type of soap. The top right corner shows which ones must be matched, and how many are remaining. During such levels, ignore other types of soaps unless they will create an easy bottle or something else useful. Save all of your moves for matching the most important soaps.
Tip 3: Remaining moves after completing a level's objective will result in random soap pieces being transformed into bottles.