Subject transparent w/Blue Screen ??

Question

I've been using Adobe CS5 now for a few months, and, of course, there is a lot still to learn.
When doing blue-screen compositing, I have tried several exposure techniques on the screen;  over exposing one stop, underexposing one stop, and exposing it exactly the same as the subject in front of it. It appears to work best if it's over exposed by one stop.
To composite the front and rear elements is pretty easy and straight forward with the Blue-screen Key, but I'm running into a problem that I just can't seem to solve.  No matter what I do, I can't seem to cancel out a slight transparency of the subject 100%.  I can always detect a little bleed-thru of the background.  I have tried viewing the foreground subject with the Mask Only option, and have made it completely white, and yet it still bleeds through when the mask is turned off again.  I have tried everything with the controls, until I'm blue-in-the-face....ha, ha, ha .
Using Ultra Key instead of the Blue-Screen Key does not seem to help either.
Could it be something weird, like the blue-screen material is the wrong color?  When I bought it, it was supposed to be for digital video, but if it is for film how much of a difference would that make?  Or, could it be something about the background scene I'm trying to composite into the shot?
Any help would be greatly appreciated.
Thanks.

Answer 1

> @ Todd: seems to me you mentioned you were working on tutorial or
something that explored the use of multiple instances of a keying effect
to get a more refined matte. Am I recalling incorrectly?
The "Color keying" tutorial on this DVD is exactly that:
https://www.video2brain.com/en/products-27.htm
I show how to use two instances of Keylight---one for the body and one for the head/hair---as well as a garbage matte and a couple of hold-back mattes for the eyes and the buttons on the subject shirt.
If you don't want to get the DVD, here is the same course online:
http://www.creativeedge.com/9780132616430
I try to not use this forum to actively advertise the non-free stuff that I make, so I hadn't mentioned this video before.
For even better keying results, I like to use a somewhat more advanced technique that Mark Christiansen describes in his After Effects CS5 Studio Techniques book: He uses a super-tight garbage matte created with either manual rotoscoping or Roto Brush (or even Auto-trace), plus a super-tight hold-out matte, leaving a narrow edge in between; and then the keying effect is only applied to that narrow edge.