keying green screen is an art. You'll need to play around with all the parameters to see what helps. After Effects may be a better tool for this and 3rd party plug-ins can also make this easier. I have a friend who's an after effects wiz who spent a lot of time with me a few years ago explaining how to do this in AE. It's a nightmare. You need to apply the key to the background, the outside edge of the key, the inside edge of the key and the inside of the key... at least that's what I remember. good luck.
and do some googling. many of the 3rd party plug-ins have demo modes to see if they'll be useful. The main thing is to light the greenscreen material properly in the shoot and have enough distance between the subject and the green background.