Project Airbus A230 help

Today I tried to do a flight from Las Vegas to Denver in the Project Airbus A320 (in America West livery) I set the altitude for 31,000 feet and the climb rate was set for 1,000. I had no fuel in the auxilery tanks, 70% full in the wing tanks, and 30% full in the center tank. Now at these setting I could not get the auto throttle to maintane my airspeed at cruise altitude (airspeed was set for .79) (It was fine during the desent)None of my other Project Airbus A320's do this. What could be causing this? How do I fix it?The panel Im using is one made by Miguel Angel Alvarez and Jorge Salas (file name is a320p11.zip)