Are you trying to get the same watermark on every page, or a different watermark on different pages?

Watermarks, in Word, are images in headers. When you use the Watermark tool, you run into things like you are experiencing. In your letter, you can copy the image created by the watermark tool from one header and paste it into the header of the page where it is not appearing.

Watermarks in Ribbon Versions of Word

Thank you for the responses, folks.

It looks like the consensus is that yes, there is a bug (or call it a software limitation if you wish) in the watermark tool. An effective workaround is to create an image of the watermark you want and paste it in the two headers. That did in fact work.

Thanks!