Note that each section can have up to three different headers (and three different footers), which will be the case if "Different first page" and "Different odd and even" are both selected (in the Page Setup dialog box or on the contextual Header & Footer
ribbon tabs). Each type of header (and each type of footer) must be unlinked separately from its corresponding "neighbor" in the preceding section.
However, if your document does not make use of either of these options, something else is going on.
I fully support Charles's suggestion that you share the document so that we can have a look.