Unfortunately it is not working on one particular post. It’s a large article that spans 5 pages. The TOC that’s up there is drawing a small subset of the headings, and they’re not necessarily on the same page. You can see from the screenshot here that that first heading is not in the TOC at all.
Is it because there’s too many headings that it’s not working right? There’s 24 main headings with 3 subheadings on one of the categories.