The level 0 bubble requires less experience, and may only be upgraded to level 1, while 1 gains 500 experience and can only be upgraded to1j. When monsters are attacked, extra experience will be stored and upgraded.
The player may have reached the experience required for the upgrade, but due to the level restriction, it is temporarily impossible to upgrade.
Please note that the game mechanism may change. If you want to know more information, it is recommended to consult the game customer service.