A family of Microsoft word processing software products for creating web, email, and print documents.
The top margin has to be big enough for the header to fit. If it is not, Word "pretends" that it is and that the word on the next page would be on the current page so it uses it as if it's the last word! I cannot verify it, but I read on this board that the issue has been fixed in 2013.
I had the same problem and all I did was remove an unnecessary Enter from the Header and the problem was resolved.
Check your Header margin and your top margin and make sure the top margin is at least as big as the header margin.
Make sure the contents of the header fit in the space allotted.
I know it's a year late but this problem drove me crazy until I found the solution--on this board!