Hard Levels
As you progress and levels become harder, the only way for you to continue is to use power-ups such as bombs in the game. You can utilize these to eliminate complete rows or even a whole color on the game board. To do this, you need to keep playing the whole board and not focus on one specific area. If you play the whole board, you'll get power-ups more easily. You need the power-ups to eliminate the squares.
Color Bombs
From the third to the seventh level, you'll find two color bombs on both sides of the game board at the start of play. Just use one of these to start the level. Hold onto the other one to use later.
After 7th Level
It is not important to eliminate squares after you reach the seventh level. Getting rid of them won't help your game. Your focus should just be on getting combos and power-ups.
Power-Ups
Keep an eye out for power-ups on the sides of the game board. These will be parts that are not connected to the game board itself. Always look out for these as you continue to play because when they turn up you can use them to move more quickly through the game.