chroma keyerno longer working???????????

I am working on editing out some green. I have two clips, one with the green in part of the clip, and another (with the cam in the exact same spot) so the "green" is no longer there and the first clip looks see through. I had it working at one time, but now the chroma keyer is not working at all (i did not change any settings or anything!) -Please help! -Thanks!