Some players have reported that they are having to download every file again when launching the game.
This is to do with a newly introduced feature in iOS 11 called Offloading Apps.
It is set to "on" by default, so if you are experiencing this issue, please turn it off. You can also take preventative measures by ensuring that you have sufficient storage space on your device.
Below are some screenshots that show you where you can find the Offloading Apps setting.