The problem isn't that the "mask can't catch up" but that the area where you're seeing the dark edge is heavily motion blurred. The result is a large number of pixels which are partially greenscreen and partially your foreground. Keying motion blur is not easy, and most solutions are rather time consuming.
If you have the Key Correct suite, "Light Wrap" can go along way towards minimizing the appearance of these edges. If not, you may need to try a different (read more fiddly and time consuming) keying technique that treats the matte and RGB values separately.
Check out this tutorial to get started:
great link, thanks. definitely learned a lot on that one. It doesn't specifically deal with blurred areas though but i do see how you can use his methods to work on the colors. I don't have the Key Correct suite, but if anyone has any other specific "what to do exactly to deal with motion blurred areas" i'd really appreciate it.
Faster shutter speed when shooting to reduce motion blur. This can create a bit of an oratorical crispness to the image but it is usually preferred over sloppy keying. An actor who understands the technologies involved and can slow his movements down.
This is what the Key Cleaner and Refine Matte effects, included with After Effects, are for.