Another observation that may be of use: If I duplicate the module, and assign the new copy to articles in exactly the same way, it now appears. But now it's stuck in the open state, with the tab displayed to the right (see the attached screen shot).
What's more, when you click on that displaced tab another copy -which I assume is the original- now pops up, which makes me think that it has been there all along, but's too low to peak up above the bottom of the screen.
Finally, if I assign the module to the top position, instead of the bottom, it displays properly on all articles... I would just change the position but it just doesn't work with my setup.
All of which leads me to suspect that the adjustment that you were kind enough to make, to get the tab flush with the bottom of the screen on the homepage, somehow pushed it below visibility in the articles.
