[otrs] Customer replies to a ticket are created as new tickets

I have an open ticket with the customer. The customer replied to an email they had received from the system (the ticket number was included in the subject line), but the email was not appended to the existing ticket. Rather it created a brand new ticket. Any idea why this is occurring? Is there a merge function so that I can place the ticket in the appropriate location? Thanks, Tyler Hepworth.