Hi Gazumphed - is rebuilding thumbnail index only available to premium subscribers, as it isn't showing in my extended menu list (unless it's called something weird)! It's been at least two weeks since the last of the notes were created, and some were even last month (as I ran out of data), and still missing thumbnails. I wondered if it might be that the file is not a supported type, but all images were taken from Google search with 'copy image' and pasted directly into the note, or sometimes pasted into Serif Photoplus and resized or cropped. These were exported as png files before importing into Evernote. If it's possible that this might be happening, I don't know how to check the file extension in Evernote.
I have tried using the Evernote online help system but, to be honest, it is worse than useless! I type in search terms and the help that I am offered bears no relation to the term I input, even when using a single word. I have tried various phrases and there is nothing helpful, hence my resorting to the kindness of other users in this community. I very much appreciate you trying to help. Thank you.