A family of Microsoft word processing software products for creating web, email, and print documents.
If the header appears correctly in Print Layout view but disappears when printing, first check File > Print to see whether it is also missing from the Print Preview. If it is, make sure you are printing the document itself (not a selection or a different page range), and verify that Different First Page or Different Odd & Even Pages under Header & Footer isn't causing the expected header to be blank on those pages.
If the header is missing only on the printed pages but appears in Print Preview, the issue is often related to the printer driver. Try updating or reinstalling the printer driver, or print to Microsoft Print to PDF to see whether the header appears there. If it does, the problem is with the printer or its driver rather than Word.
If the problem started suddenly, also try starting Word in Safe Mode (winword /safe) to rule out an add-in, or copy the document content into a new Word document in case the original file has become corrupted.
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hth
Marcin