Make sure you have updated to the latest version of the app. Resetting game data will help remove any bugs causing this issue. You can double tap the curved yellow button on the main page of the app to reset game data. You can also reset via the settings app on your device. Go to Settings > Dr. Panda Town > Reset save data on next launch. Toggle to on and then go back to the app and launch it again.
Please note that deleting game data will erase all the progress you've made in the game thus far.
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