OK, problem solved.
Watermark ONLY works on new images.
You CANNOT add a watermark to an existing image by editing the properties regenerating the images and/or editing the ad.
The solution to that would be to download the ad images to date, watermark them using a watermark app and reupload them.
CORRECTION - It seems that you can modify the watermark if the ad image has already been watermarked (OK, go figure) by eg changing the watermark image properties and regenerating the images. In my case I kept it bottom rh corner but added padding in the png on its bottom and rh side, effectively moving the wording.