There are two common reasons:
Simulator bays are unavailable
If lessons are tied to simulator bays, and none of the bays are available for booking at the times you look at, the calendar will show no available times.
Minimum notice settings
If the instructor has set a minimum notice that is longer than the time window you are viewing, the instructor will appear fully booked.
Example: Minimum notice is 24 hours, and you are viewing earlier times. No availability will appear.