Hi,
if I understand the game well, you are correct.
When an airport does its daily check of passenger demand, it uses the flights that are available. In other words, flights that are already entered in the ORS.
Every airport has a "black spot". That black spot is the period between "Enter flights into the ORS" and "Process travel desires" of any particular airport. Every 30 minutes the system enters the new flights (of the past 30 minutes) in the ORS, so the black spot of an airport is always less than 30 minutes. But flights that take off during the black spot only book passengers twice.
However, I have noticed the same thing you have noticed. Some flights should already be in the ORS but they still don’t get any bookings the first day. So I asked the same question that you asked. The answer I received, came down to "when flights are in the ORS, passengers can book them".
I believe what people tell me, but I also believe my own eyes. So the answer did not really satisfy me 
Later on, I have thought of a possible explanation. If the passengers that book your flight are transfer passengers, they cannot book their tickets the first day because the transfer flight they need is not yet in the ORS. But perhaps you can come up with another explanation.
Jan