Books App not downloading over cellular

Hello,

I find that I can now only download ebooks in the Books app over wifi. Whenever I'm trying to download them using a 3G connection, I get the little circle for downloads but nothing happens (even though the files are only a few megabytes).

I think I've enabled cellular data in the three places I need to: "Settings -> iTunes & App Store -> Use Cellular Data", "Cellular Data -> iBooks", and "iBooks -> Cellular Data". (Don't know why the last two are in two different places.)

It used to work before, but not in a while... Probably because of an update, but why would Apple create a bug that wasn't here before?!

Many thanks in advance for any help.

Nemesia

iPhone SE, IOS 13.7