Not sure why that would happen unless you are low on memory.
A workaround if you really need to get it uploaded right away would be to export the book as JPG and upload the JPGs with Booksmart. I actually do that all the time as I use LR to get the page numbering but then want to use Booksmart to get a different template for the covers...works fine.
Hope this solves your problem.
If I download my book from LR5 as JPG's do I have to rearrange everything in booksmart?
TThe only thing you have to watch is the file naming.
IF they get renqmed in LR, you might have to manually reorder/rename the single digit ones if you use my system, which is to load them into book smart and sort by file name and then auto load them into the book.
iit can be done in lesa time than this took to type
What error message? What operating system and version of Lightroom ? Does exporting to PDF work?