When attempting to download a purchased riff on the Sync app, you may encounter a message that "A network error occurred." This is typically seen in iOS, due to the way iOS manages content that can be downloaded (as opposed to system storage).
When a downloaded riff has not been watched recently, or if your iOS device has started to get low on storage space, it is possible that iOS will delete the file, which is stored temporarily, from your device cache. If you find that a previously-downloaded riff will no longer download from the Downloads screen, try deleting the download entry by tapping the "Edit" link at the top, tapping the X for the riff you want to delete, and then try downloading it again.
If a previously-purchased riff that you have NOT downloaded fails, it is likely because of a server change. In this case, reinstalling the app should solve it.
If none of the above options work, please submit a trouble report through the app's "Report a Problem" feature. The included log file will help us understand the root cause of the problem.