You formatted the only Section in the document with a different first page. Then you added a section break so both Sections in the document have a different first page.

So if you add a page number to the first page footer of Section 2 then that is the only page the page number appears on. You should put the page number on the subsequent footers if that is what you want. Or turn off the different first page for the second section before you add the page number.

If your first section is intended to be only one page then you can avoid the different first page completely.

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Andrew Lockton
Chrysalis Design, Melbourne Australia