This could be due to a few things:
1. It can happen if you have an unstable internet connection.
2. It could be that the game has not been updated to the latest version
Please check for both of these possibilities.
If the above has not fixed the issue, please uninstall and then reinstall the game.
Important!Please make sure that your game has been saved to the Game Center or Google Play before uninstalling so that you do not lose your game progress.