most cases is your current available memory at that time. sadly, there are no actual way to check available memory(to check is to use and flush, there's no such thing as just checking). If it's just memory problem(not storage), then it should be ok when you relaunch the app and have no other apps running along. This game uses quite a lot of memory during launch and when viewing sidians. In some cases is that your connection is not stable(not fast, stable) during downloading the file, sadly, the only safe way for this is to re-install. Not sure deleting the files would make the app download again or will get you into troubles.