Problem with Chroma Key

Hi everyone.
I've tried to make a chroma key using a green screen and a photocamera (I don't have a video camera, this is the possible first problem).
The screen is uniform ad flash green, the light was quite good but, the final results is full of, I don't know how to decribe, but seems as video glitch.
Every border are really not sharp. I've tried a lot of setting with variance and I've also combined more than one chroma key.

Some of you have ever used kdenlive with chroma key?
Have you achieved good results?

Thanks in advance.

It would be easier to diagnose your problem, if you could put an example picture somewhere to see (a single frame would be enough, as it was before keying, and what you get after).