The user may be directly replying to that original email which he used while creating a ticket using email piping. He should reply to the ticket email notifications which are sent to him by our plugin. If they reply to the original email then plugin does not get a ticket ID and hence the reply did not get imported into the ticket and instead creates a new ticket.
To achieve this you need to use the external cron method. You have to set cron in your hosting control panel. You can set up the external cron directly to the wp-cron.php file. Please see this article for more information. This will help you to set up an external cron. After that go to Support > Settings > Email Piping > Other Settings > Cron Execution Time and set it as per your requirement.
Please try to configure your email piping email in any email client like thunderbird, outlook, apple mail, etc. If it is successfully configured in the email client then please use the same details to configure email piping.