Q: Problem with Yosemite Photos--Photo Book

I just upgraded to Yosemite and am using Photos to create a photo book as a gift. There have been lots of issues with Photos and it's taken me weeks to get this book right.. But I've finally resolved everything  and am ready to order the book. However, when I click on Buy Book, I get a message that says there are empty spaces that need photos. However, there are NOT--I've reviewed the book several times and previewed it in a PDF. It looks totally fine, and Apple Support told me that if it looks good in the PDF, then it's OK. But I can't get past this message. Does anyone know of a way to get around this problem? Thanks!