The simplest solution is to open the header - Type CTRL+F9, then between the
{} brackets type Filename \p. Press F9 to update and toggle the display,
select the field then ALT+F3 and save with the name Filename and path.
Confirm replacement of the original if prompted.
What has probably happened is that the entry is no longer associated with
the paragraph style used in your header. If the entry is missing altogether
from the autotext list (insert > autotext) then the usual causes is saving a
document as normal.dot.
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Graham Mayor - Word MVP
My web site
www.gmayor.com
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