I cannot seem to add a watermark to more than the first page if using placeorder = background, only seems to work if it is set to foreground. The problem with 'foreground' is that all of the text in the document is "greyed" where the watermark is being displayed. The background displays fine, but only on the first page.
You may have coded something wrong, or your PDF file has a white rectangle on the background - in such cases, the background watermarks will be invisible.
You can check the file by opening it in our Viewer or Adobe Reader/Acrobat and turning on the "Display transparency grid" option.