Chroma Key Problem in Final Cut Pro X

In the chroma keyer, I lose part of the foreground image. The keyer is fine and looks great when i enter the keyer, but the next day the mid-secton of the person in the foreground is gone and showing the background. Oddly this only appears in certain clips, not all. All were shot at the same time and in the same way with the green screen. The matte is showing a clean image with clear defined black background and white foreground. There's no gray. Using FCPX 10.5.4. Thanks for helping a FCPX rookie. An image is attached.