By default, we do not show the page number on the first page of a new chapter. This is just a standard publishing convention.

You can configure this setting by selecting the Custom theme here:

...making sure to replace YOUR_BOOK with your book's unique web address.

At the bottom of the page, tick the box next to:

Then, click the blue "Update Theme" button.

The next time you create a preview, you should see the page number on the first page of each chapter.