This happens when the camera is not able to recognize your face. In AutoFly mode the AIR PIX is using facial recognition technology. This works by tagging your forehead, eyes and chin to lock in to take a picture. If the AIR PIX cannot recognize your face it is programmed to gently land. To ensure the AIR PIX can recognize your face, make sure you are in a well-lit area. If you have too much light behind you (a lamp, a sunny window) this light will cast a shadow around your head and make it very difficult for the AIR PIX to find your face. If you are wearing a hat it may be blocking your forehead, this can sometimes make it difficult for the AIR PIX to lock in on your face.
This whole sequence is pre-programmed to take about 15 seconds to complete. When tossing your AIR PIX, try not to toss it too high, try to toss it out in front of you so it can find your face quicker. If it is too high it will spend too much time trying to find your face. If it can't find your face this is when you will see it slowly drop down to the ground. It can not fly back to you because it "can't find you".