I have imported a color picture and trying to use the adjustment brush to turn the background black and white and it isn't working. I have Lightroom 5. Any suggestions?

Hello - I have Lightroom 5.  I have imported a color photo.  I am trying to use the Adjustment Brush to change the background to Black and White and leave a portion of the photo in color.  When I begin painting the photo with the Saturation at -100 I don't see any changes to the photo.  I push the "O" key and still nothing.  I have tried to reset the photo, restarted Lightroom and still nothing.  Please help.