By default, Microsoft Word only includes Heading 1, Heading 2, and Heading 3 in an automatic Table of Contents. To include deeper levels like Heading 4 or Heading 5, you must manually adjust the "Show levels" setting.
Steps to change the levels:
Go to the References tab on the Ribbon.
Click the Table of Contents button and select Custom Table of Contents... (at the bottom of the menu).
In the dialog box that appears, look for the General section at the bottom.
Next to Show levels, change the number from 3 to 5.
Click OK. When Word asks if you want to replace the existing table of contents, click Yes.