There's no way to book you onto a thru flight when you've already flown the first half. DL123 XXX-ATL, DL123 XXX-YYY, and DL123 ATL-YYY ate all different flights even though flown on same aircraft. Once in ATL there's no way for them to book you on the DL123 XXX-YYY, only DL123 ATL-YYY.