That really depends on what's going on in your document at page 20 and thereafter. Perhaps you have added a Section break at that point and the header isn't linked to the previous Section?

As discussed in the linked thread, the code was written to apply the watermark to the first Section only. If the Section headers are linked, a watermark added to the first Section will appear in all Sections.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]