Just in general, the availability of "saver" awards versus unrestricted awards may be affecting the miles required. Most airlines will have a saver award (that can be purchased with less mileage but is heavily capacity controlled) and an unrestricted award (for a higher cost in miles you eliminate most restrictions). If the saver awards are gone then you would be relegated to the higher mileage award to get your tickets. Sometimes airlines will open additional saver awards as the flight gets closer (based on ticket sales and other factors), which is why the agent may have told you to call back later.