Did you upgrade to 5.7.1? There was a bug at certain versions that made Lightroom omit the watermark for certain sizes in export. Also when you use a text watermark, make sure that you export to a reasonably small size such as 1000 pixels on the long side. If you export to full resolution often the standard copyright watermark is so small that many people would never notice it. It will be absolutely tiny in the bottom left corner and only visible if you zoom in on the exported file.