When keying, it's sometimes hard to tell how good a job you've done when you view the keyed layer against its composite background. Detail in the background might hide flaws in the key. You can view the key more accurately by temporarily replacing the background layer with a garishly colored solid. Make the solid color the opposite color from the original screen color. For example, if the original image was shot against a green screen, try laying it over a red solid. Be sure to check the key by moving the playhead to check the key at a few points in time.