davidfor!!
Apparently, my problem was two-fold:
First, I did have the TOC window hidden as you speculated. I feel like such an idiot!
But then, trying the 3 epubs again, the TOC only worked on two of them. Turns out the third epub was missing the "toc" attribute in the opf spine. Once I corrected that (and unhid the TOC window), the TOC appeared.
In case this is useful to anyone else:
* If the TOC icon is blue and the book text wiggles but no TOC appears when you click it, your TOC window is most likely hidden, per davidfor's post above.
* If the TOC icon is gray, clicking it does nothing (no wiggling text), and you're positive there is a TOC (in my case, the TOC appeared in ADE so I knew it existed), check your opf and ncx files for errors.