This is unfortunately a known issue, that some players are experiencing.
You can try the following troubleshooting tips:
- Make sure you are running the latest operating system version on your device
- Free up your device memory by closing apps running in the background
- Increase your storage memory by deleting files/apps from your device you no longer need
- Restart your device
- Force quit the application and restart
If none of the above work, you can try deleting and reinstalling the game. Before you do that, make sure you have saved you progress by connecting to Google Play (Android) or Game Center (Apple iOS).