As part of my project, I use the current schedule to obtain flight status for a sample of flights from the schedule. Every now and then, there is no status available. For example, for this flight:
It appears the majority of these flights are position only. Your account does not have position only enabled, therefore, those flights do not show up for you when queried. You can adjust this setting under #6 at https://flightaware.com/account/manage. As far as them showing up in the schedule, they are still scheduled flights and do operate.