Many things can factor into why a course did not fully download. You can delete the downloaded files by clicking on your username in the top right and going to Storage.

On the Storage page, find the course you’re having an issue with and click the Trash Can to delete the downloaded files. Now you can start the download again.

Please note using the Download All button can create a bottleneck if your internet is not fast. HD files are 4 to 5 times larger than SD files, so downloading SD files can make the download faster and more complete.