Originally Posted by paul-b
Yep. It was file size. As an EPUB it was around 11 MB.. but when uncompressed it was in excess of 50 MB. I split it into two Volumes.. and it works perfectly now.
A bit disappointing though. I really want to publish it in one EPUB, like the paper version of the same product. (it's a really big technical reference)
I do not believe it was necessary to make two volumes. It should have worked if you just split the file itself into smaller pieces. It is likely, not that you have split it, that you could assemble it in one ePub and it would work.