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 is not the latest version
Please check for both of the possibilities.
If the above has not fixed the issue then please uninstall and then reinstall the game.
Important!
Please make sure that your game is saved to Game Centre or Google Play before doing this so that you do not lose your game progress.