Background image in Main Banner Block does not display in resposive mode

My July 2016 child theme background image displays in desktop mode but not when viewed in a mobile emulator.

Nor does it display when I use the smartphone/tablet test functionality in Appearance > Customize.

The same image was visible when used with other child themes.

I do have the Theme Setup > Responsive design option turned on. The page is set to box design, just as with the previous child themes that displayed the image OK.

What am I doing wrong? Is this July 2016 child theme OK to use with the responsive Directory Theme?

Thanks
Larry