Most travel agents susbscribe to one of the major booking systems. These list flights in a certain order, which sometimes has to do with who owns and runs the system!
Also, an agent may get a higher fee for booking you on one carrier over another, so it is in their interest to tell you that there is only one flight a day.
BA cut their commission fees to agents, think ing that it would save them money. Some claim that the reduction in bookings from agents is due to them putting the business elsewhere, in order to get better commission.
If you want a complete listing of all flights, you have to pay for it. You can subscribe to a CD or on-line:
Official Airline Guide