There are some circumstances in which the combination of the source creation application and the printer driver will draw a white rectangle behind the text of the PDF document page. In this case an underlaid stamp will be obscured by the white rectangle. The work-around is to stamp the document with an overlaid stamp of outline text. Choosing a shade of gray or other light color will further help to prevent obscuring effects of an overlaid stamp.
This circumstance also occurs if you stamp underlay on any page that is a known image. When viewing the document, Acrobat draws the stamp first, then the image on top of it, making it look like the stamp flashed and disappeared.