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It seems when you make a match 3 it leaves behind a grey square which cannot be matched with anything? If that is the case you are kind of punishing the user for making lots of matches. It might be better if the grey squares could be matched as well. Also, not sure where Tetris comes in other than the gravity...?
Getting a whole row of grey blocks may be too hard..? I would say just 3 grey block should be enough same as with other blocks.
Maybe you could start level with some colors squares on scene to the reaction time either faster and you increase the difficulty a bit.
User needs to make color block matches or grey blocks in rows to complete the level.
In other levels I have added some static blocks to increase difficulty.
This is level 13 with static blocks: