There are many possibilities / reason for that
- Ticket change will trigger re-issuance (change from transit to stopover)?
- Agent is too green for handling complicated ticket
- Fare rule restrict on specific flight
- Calling the wrong guy: PLEASE always call wherever you purchase the ticket before your first sector commence (online = eService Center, NOT Reservation)
There are many possible issues / possibilities around that and the best for you is call a lot and ask a lot....