Check if you are using the correct stage requirements; some stages will require a specific type of Pa Pa monster or in other words, you need to level up to play a particular stage.
Check your internet connection, try playing the game in places with better connectivity or try re-installing the game.
Hearts regenerate with time. You can also purchase them from the shop.
You can make purchases based on your gems and LOLL point availability. In the event you don’t have any, you can add purchases with your Dialog mobile credit. To do this, you need to be registered with LOLLAB.

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