If the demon flying at the forefront noticed what happened behind, it would have been able to escape. Although Andrea could easily shoot down a floating balloon from the air during previous live-fire drills, she noticed that it was much harder to predict the movements of a living being.
Once she pulled the trigger, the bullet would dart toward a definite position. She could not control the situation if the target left that position on its own. Thus, if a living being became aware of the danger and suddenly decided to change its path, her bullet would most likely miss the target.
She never missed a close target and always knew the result shortly after an attack was launched. Now, however, she needed to wait patiently for the result as the bullet had to travel five kilometers to hit its target.