The user may be directly replying to the 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 the plugin does not get a ticket ID and hence the reply did not get imported into the ticket and instead creates a new ticket.
Also, please check the allowed email type at Dashboard > Support > Settings > Email Piping > General > Allowed Emails > Select All Emails.
Category: Email Piping