With plain text, if you prefix headings with # then it will turn them into chapter titles as described here: Readlang text markup - Readlang Blog
With epubs, Readlang extracts the text and tries to identify the chapter titles, but it doesn’t always work. If you send me the epub you are using to steve@readlang.com I’ll take a look to see if I can improve the importing code to identify the chapters.