Updating watermarks on thumbnails

After upgrading Nextgen, I had a problem in that the default font size for the watermarks was set too small. I have now changed the watermark font size and restored the original images which removed the old watermark. Then I have set the new watermark which now has the correct font size on the main image. Now I have always found that, to get the watermark to show on the thumbnail, it is necessary to edit the thumbnail. No changes are necessary, just drag the selection box to cover the whole image and it will apply the watermark to the thumbnail. However, after restoring the image from backup and changing the watermark to the new font size, the thumbnail still show the old watermark and I can’t change it. Editing the thumbnail has no effect and the old watermark remains. I’m using the very latest versions of WordPress and NextGen

https://wordpress.org/plugins/nextgen-gallery/