We recommend that you use wifi when downloading a book offline, and make sure it’s 100% before you start to listen or disconnect. Make sure you have a stable signal and no firewall to interfere with the download.
For downloading around 5 books you should have 2GB free at least.
If you have an Android device make sure you give the app full permission to work. Google has changed it's guidelines and you now need to approve this. It is not anything we store or use, but we need it for the app to work.
If you have granted permission but it still does not work, go to Settings in your device and give the Storytel app permission there.