Several table of content's items not showing in the hyperlink menu

Question

I want to hyperlink to a certain chapter somewhere in my book.

It works fine for some chapters but for some reason Chapters 1, 2, 3, 4 and 7 are not shown.

Here's the original table of content.

What's seems to be the problem?

Answer 1

I realized the problem was that missing chapters' titles didn't have proper styles.

The problem was fixed when I made all the missing chapters' titles Heading 1.