Blurb button missing from Book Module in Lightroom Classic

I can export some Books to Blurb, but not others (see attached clip - the Blurb button is replaced by a PDF button). I can't find any settings that cause this difference or work out any other reason why it happens. The Books I can't send to Blurb I have to export as PDFs instead and then use Blurb's PDF upload which ofted causes problems. I use LrC 13.1 with MacOS 13.6.1 on a Mac Mini M2 Pro - but I had the same problem with earlier versions of LrC on earlier Macs, so it's not new or specific to this setup. Any help would be appreciated, thanks.

[Moderator Note: Edited the thread title]