I have several clips in a timeline of someone speaking in front of a green screen. I have selected a still image and dragged it below all the clips in the timeline. I then select a timeline clip and double click the Keyer effect. Some of the clips work as expected. Some of the clips show the background image through the person talking leaving the green screen alone. I do not believe there is any difference between the clips that are working and the ones that are not. What am I missing?