Make sure that every section header/footer is set to "Same as Previous"
("Link to Previous") and that numbering in every section is set to "Continue
from previous section." Occasionally numbering will get restarted in a
multi-column section in the middle of a page. The only (easy) way to access
the Format Page Number dialog for that section is through Insert | Page
Numbers... | Format, but use this carefully and exit with the Page Numbers
dialog with Close rather than OK so you don't insert another number.