The links you receive via email when your first make the purchase will become invalid after 48 hours for security reasons. If you are trying to access your downloads via the links received by email after this time, the download will fail. Another common reason the download may fail (whether after 48 hours or not), again for security reasons, is that you may have simply “left clicked” the link.
In either case, all you need to do is log back into the account you created on this site when you made your previous purchases. (Go to the “Hi Guest” link on the righthand side of the menu and then “Login”). You’ll be redirected to a page where you can re-access all your successful purchases. Once there, make sure you “Right Click” and “Save as…” in order to successfully initiate your download.