1. Make sure you have an active internet connection (data or wifi).
2. Tap on My Courses (Android) or Synced (iOS) and see if the course you are trying to download already appears there. If yes, delete the course by tapping the 3 dots icon right next to the name and select Delete, if you're and Android user, or by swiping left on the course cell and tapping the red Trash icon that appears, in case you are an iOS user. Then search for the course and try downloading it again.