Please make sure your device meets the game's system requirements. You can check the requirement on the game page on the iTunes Store.
Try to reboot your device and run the game again. It can resolve the problem. It's suggested to reboot your device from time to make it work smoothly and fast.
Here is how to reboot your device (turn it off and on):
- Press and hold the Sleep/Wake button until the red slider appears.
- Drag the slider to turn your device completely off.
- After the device turns off, press and hold the Sleep/Wake button again until you see the Apple logo.
Also make sure you have the latest version of the game:
- Run the App Store app on your device
- Make sure your are signed in under the same Apple ID that you used to buy the game
- Go to the Purchased tab and see if the game is on the list of available updates.
Tap the Update button next to the game to download the latest version.