Ah, well... I think I figured this out, from a
seven year old thread in the forums.
So, I guess that when you first load a book onto the device, the series info can't be sent. Then you have to send the book again, and the series info is sent?? Something like that. I don't understand how it worked before (or why the data was lost), but this isn't the first time I've sent books to the device multiple times, so maybe I just lucked into it before.