Download link expired & Delete Permanently

I got problem with my link, at the beginning everything are good but suddenly I got complain from my subcribers the link i given are expired

I opened my directory (domain), than I don’t see the file (what that suppose to share with my subscribers) in the directory any more (wp-content/uploads/download-manager-files/file)

Why this can happen??

Than I remade the link and I don’t know why I cannot Delete Permanently the old link

Why the function are not working???

What are the best action to prevent this problems happen again especially “download link expired”, because this are so critical problem!!