Hi,
First, we need to understand what does "no longer receive the download link" means.
Do you mean that they still receive the normal email, but there is no download link in it ?
Or they don't receive the email at all ?
Do you see the email if you check in the Customers>Emails history menu, which logs all the emails being sent by HikaShop ?
If you do, do the email contains the download link ?
If the email is in the history, with the download link, but the customer is not receiving it, it means that the email has been flagged as a spam. In that case, you can use
www.mail-tester.com/
to test the spammyness of your emails. It will give you information about what is problematic and what you can do.
If the email is not in the history, is the status of the order changed to "confirmed" after the payment ?