Ditto Primatte, also use the feature in that same collection in Motion (if you have it) that adds in spill killing correction to the edges. It will add magenta to edges on green screen and yellow to blue screen, to kill spill. Don’t overdo it.
Use garbage mattes so you don’t have to worry about a clean key wall to wall but just around the body and hair.
A trick I have done before on short 30 second spots with troublesome footage is to pull the keys using photoshop. Export>quicktime conversion> sequential stills> targa format to a folder. Open the folder in photoshop, under the tools for the magic wand and selection, use selection by color. You generally only need to do this to the first frame in the stack. If you record the actions and save the action, you can apply that to the entire folder automatically and get something pretty amazing back in minutes. Re-import the stills to your timeline, back in business. Targas contain alpha channel info, at least in 8-bit, you can leave the alpha blank or fill the cut-out with a better overall chromakey color and then do the keying in FCP again… or whatever.
(addendum-dum: you can get in trouble importing the stack of stills two ways: by not using leading zeros, i.e. frame 0001, frame0002, etc, which can put some frames out of order, and by forgetting to set the preference in your import dialogs to a duration of just one frame per still you import.)
Something that happened to me once: We found it impossible to blue screen a lady with blonde hair one time. Asked her if she’d just shampooed and what brand. Some brands add a bluing agent to the soap, it turns out.
Unless specifically using Ultimatte, next time you shoot, use a real gelled backlight on the set to help kill spill contamination and I bet the key will work easier even in DV’s 4:1:1 color space.