By default the request delimiter will be ##. When you reply to a ticket, the subject line should contain ##ticketid##. Only then the reply will get appended to the parent ticket. If you want to change the delimiter you need to update the same in the database table.
update GlobalConfig set Paramvalue=’@@’ where Parameter=’Title_Delimiter’;
The above command will set the delimiter to @@ in all tickets. After executing the above query you need to edit the notification rule and set the new delimiter there also and save. If you want single delimiter like @ticketid@ then update like below,
update GlobalConfig set Paramvalue=’@’ where Parameter=’Title_Delimiter’;