I think I figured this out - perhaps it will be useful for someone else.  The
2'nd monitor at my desk has a different resolution and orientation than my
laptop primary monitor - in particular it is 1600 pixels high (portrait
orientation) compared to my laptop's height of 900 pixels.  When I make a
presentation on to the bigger monitor, it adds black bands above and below
the slide.  This height difference is apparently not taken into account in
the preview window of the presenter view application.  The slide and top and
bottom bands completely fill up the screen and leave no room for the speaker
notes (they are probably right off the bottom of my screen hiding under the
keyboard).  I think this is a bug in the presenter view.  When I use
presenter view with the real projector (landscape orientation), however, it
seems to work fine.

I was sure that I had used presenter view on my laptop / portrait monitor
combination before, but perhaps I was mistaken.