Fresh copy of theme loaded. myatu - tried your suggestion of:
Sure enough, content 'disappeared'.
I then compared (using Firebug) every bit of HTML and CSS in the two sites. This CSS below was present in problem site, absent in OK site. It said it was to be found in style.css line 2180.
body.full-width, body.full-width #inner-wrapper
So I checked both style.css files. Identical - and both had the piece of code above. Finally, I checked every Theme Option in the Canvas theme on both sites. And the only difference was I had ticked the boxes for Enable Full Width Header and Enable Full Width Footer in the problem site and had not done so in the OK site. So I disabled both those boxes on the problem site and tried your Background Manager again. Still didn't work.
Even to my non-tekky mind, it's clear that there's a problem with a wrapper obscuring the background. But I cannot see how to fix it.
I have been using Woo Canvas for years. The two sites both have the latest version Canvas 5.5.3 and WordPress 3.7.1. There should not be any difference between them but there obviously is. And it's interfering with plug-ins like yours - and also slider plug-ins.
Woo support is no longer free and I can't afford to pay one of their code monkeys. I'm a middleaged woman trying to learn coding on my own and problems like this stump me. I wonder if there are others using Canvas who are encountering this problem.