The correct way to add page numbering is to insert a PAGE field in the document header or footer. Doing things the way you are makes the document a nightmare for any further editing.

The only line that becomes left-aligned is the last line. That is normal behaviour when you end a paragraph, which is exactly what pressing the Enter key does. Look at any book with justified text - the last line in each paargraph doesn't stretch all the way across. In all probability, the first time you press Enter, the paragraph break is being added to the end of the previous line - which is on the previous page. That's why nothing moves on the new page the first time.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]