My headings are H1s and nothing is listed
H1 is switched off by default, and that is usually right: on most themes the only H1 is the post title, so a contents list built from H1s would have one entry — the page you are already on.
If your content genuinely uses H1 for its sections, tick H1 under Settings → Heading levels. It is worth fixing the content instead if you can, because several H1s on one page is a real accessibility problem independent of this plugin.
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