Kobo epub ToC Issues?

Hi, I wonder if anyone can help me. I have a kobo H2O that I use to read epubs. On some of my books when I bring up the Table of Contents, there is a tick mark next to the chapter I'm currently reading. However, this doesn't seem to work on all of them. On some, the tick mark stubbornly remains on the first chapter heading and doesn't advance when I move to a subsequent chapter. The chapter links work correctly; Sigil and the Calibre book editor don't show any issues with the ToC; I'm using a flattened (non-heirarchical) ToC.

Does anyone have any ideas how I can format my Table of Contents such that my Kobo always knows which chapter I'm currently reading?