- Subject lighting is dim or subject is moving quickly.
- Subject and background are similar colors.
- Subject is backlit or very brightly lit.
- Subject is very large or very small.
- Automobiles, bicycles, people, or other objects are moving behind the subject.
- Trees, foliage, or flowers are between the subject and camera.

Try altering shooting conditions so the camera will be able to detect the outline of the subject more clearly and shoot again.