To overcome some internal errors of the application, it's advised to keep the game up-to-date at all time. You won't lose any progress when a new update is installed. Also, to be noticed that if you uninstall the game, you won't lose any data and you can continue with the game as soon as you install it again. To advance to another tank, you should have at least level 5. After all this, I suggest to reinstall the application to see if your problem gets fixed. If not, you might want to take your phone to a specialized service for additional investigation.