There can be various reasons for this to happen: Some of them are listed as follows. Do check each one and verify it at your end.
Most importantly check if your plugin has been updated to the latest version.
Check 1:
Do you have any SMTP plugin installed and activated on your website?
The plugin developers suggest the use of an SMTP plugin. The default email system built into WordPress is not always very reliable. This plugin drastically improves the process. WP Mail SMTP
Check 2:
When using a Gmail account to configure SMTP, the emails will still be sent from your site and the recipients won’t know the difference.
For it to work with Gmail, you have to enable it in your Gmail settings. Go to Settings > Forwarding and POP/IMAP, then make sure that POP and IMAP are both enabled.
Check 3:
Did you add the {download_list} tag to the purchase receipt email in Settings > Emails?
Here is an example for the same with {download_list} in it.
eg.
Hi {name}:
On {date} you purchased the below item or items for a total of {price}.
Please click to download:
{download_list}
Regards,
{sitename}
Check 4:
There is a bug in the plugin which should be resolved in future updates. It turns out that including dashes, -, in the download files names breaks the email.
Download files names with spaces and no spaces, and both working fine.
Check the following scenarios and let us know whether your issue is resolved.
