PS:
"(using PAGE or WORD PROCESS mode..)"
In a Pages word processing document, a Section may include any number of pages. The watermark object is placed on the Section Master, and will appear on all pages in that section. If you enter a section break, or otherwise create a new section in your document, Pages in the new section may not have the watermark.
If it is missing from the next section, you can place the insertion point on a page that has the watermark, then go Arrange > Section Masters > Make Master Objects selectable.
Then Select and copy the watermark object, go to an empty page in the section missing the watermark and paste.
Align the object on the page, then, with the watermark object selected, go Arrange > section Masters > Move Objects to Section Master.
In a Pages page layout document, each page is a separate section, and Section Masters do not exist.
Your watermark object may be pasted onto each page of the document, and placed in the back layer.
The easiest way to do this is likely to create the watermark using the same process as above, and making it the first object to be placed on the first page.
Select the thumbnail image in the left sidebar and press command D to duplicate the page (and the watermark object).
Regards,
Barry