A TA also once explained to me that their computers can't handle a request to check a bag on to a flight that is not the next instance of that flight number. So, for instance, if there's a QF94 LAX-MEL departing in 5 hours and another departing the next day (29 hours), the computer can't understand that you want the one in 29 hours. (I have no independent knowledge as to whether this is true, but this TA seemed quite knowledgeable.)