Sigil builds the TOC using heading tags (h1, h2, h3, etc ... ). If the chapters in the calibre-created epub don't use h tags for chapter headers, you won't be able to automatically build a ToC with Sigil.
So to answer your other question ... to "get it to work every time," make sure your epub uses proper heading tags for chapters before trying to build the ToC with Sigil.
|