In my experience, an escort's availability does not just depend on her work schedule, but also on her desire for bookings.
Some escorts at agencies seem to want as many bookings as they can get, and they are flexible in terms of availability, location and duration. Some escorts seem to rarely be available, and when they are, they are very selective about the particulars of the date (location, duration, etc.). It seems that the more appealing escorts are overrepresented in the "rarely available" group.
Most agencies list their escorts on their websites in an order that matches availability and/or desire for bookings.