By default, paragraph styles Heading 1, Heading 2 etc are pre-set in Tools > Outline Numbering for use as chapters.
Styles Contents 1, 2, 3 … are normally used to format the appearance of the entries in the Table of Contents.
It’s the Level number allocated to a paragraph style (in Tools > Outline Numbering) that determines what appears in the TOC, in conjunction with the “evaluate up to level”, when you right-click on the TOC and select “Edit/Index Table”.
By using the Contents 1 style for chapter formatting, you may need to adjust Tools > Outline Numbering to reflect this fact or re-jig using the default settings?
EDIT TO ADD:
Another thought: I was caught out with missing TOC entries when I used paragraph styles which simply created Act 1, Scene 1, Scene 2 …, Act 2, Scene 1, Scene 2 … headings. I didn’t add (or need) any heading tecxt of my own. I discovered however that you HAVE to enter some text (manually) at each heading in the document (a space character will suffice) otherwise the heading won’t appear in the TOC.