A family of Microsoft word processing software products for creating web, email, and print documents.
Based on your description I would guess that you have odd and even pages setup on the document, but the page number has been added only to either the odd page footer or even page footer. The other possibility is similar and involves multiple sections in the document.
For either of the above scenarios I suggest you open the footer on the first page of your document. Then using the Header and Footer tab on the ribbon, go section by section (there are buttons for Next Section and Previous Section navigation on this tab) and verify that each one contains a page number in the footer.
Hope this helps