In order for your subtitle to appear on the page, you’ll want to make sure that subtitle is selected as a visible element in your theme settings.
In your theme settings under the Chapter Heading section, you have the option to customize the header information displayed in each chapter. Within this section you can select the details you wish to appear in the chapter header, such as the chapter number, title, subtitle, and/or image, by checking the corresponding boxes.
If you need more help with creating or editing your theme, I would recommend checking out this tutorial: https://www.atticus.io/create-a-custom-theme/