It seems as though if an agent replies to a ticket via email while the status of the ticket is "waiting" it doesn't import the email. For instance, if an agent replies to a ticket via email that is "active" then this email gets imported into ST and the ticket status changes to "waiting." But if the agent replies again via email to this ticket before the customer has responded, so the status is still "waiting," then it doesn't seem to import this second email from the agent.