Problem with Chroma / Color key, stops working.

Hey all, got a problem with the chroma / color key

I set up a scene with a white background, and set up the chroma key to white, so the part that was white is now transparent. But sometimes it just refuses to work!

Same with color key! How do I fix this problem?

Why are you using a color keyer to key out white? Why not use luma key?