Look at the paragraph formatting of the heading that gets moved to the next page. Most upper-level headings have the paragraph formatting set to "keep with next" and this is appropriate.

If that is what is happening, try putting an empty body-text level paragraph between each heading.

Headings are not intended to be the only text in a document. If you want them to be the only text, remove the formatting for "keep with next." If not, use the empty paragraph until you fill in some body text.