I Have Lightroom 5 and an iMac
2 weeks ago I successfully uploaded a 182 page book to Blurb website.
Today I tried to upload a similar book ( the only difference was the following actions I took) . The book would not upload see screen shot. I contact my internet provider - confirmed that my internet speed had not dropped.
1.changing some photos from page spreads to single photos
2. removed a number of photos.
3. deleted some pages - ended up with a 172 page book
3. changed the numbering from the side of the of photos to under the photos.
4. Justified the printed pages to the extreme right.
I did not consciously change anything within the book module.
As an experiment I even tried a uploading another book I had uploaded last year . Also would not upload.