Welcome to the wonderful world of extractions...if your foreground is a similar color as the background, you are almost always going to have a difficult time separating the two. You will likely need to hand trace (rotoscope) the edges that the software cannot detect. You can try using more sophisticated extraction tools/plugins in something like Nuke, After Effects or Photoshop...but similar colors will almost always be a challenge.
Sorry, and good luck!
-C