Q: I exported a dozen photos and some of them are missing the copyright watermark. Why?
To add the copyright watermark, you need to select it during the export:
Lightroom: Add Copyright Watermark
And Lightroom uses the value of the IPTC copyright field for that. If some of your photos don’t have the copyright watermark, it’s because Lightroom did not find the copyright information in the IPTC copyright field for that photo.
Just edit these photos and add the IPTC copyright information.