I have a persistent but intermittent problem when uploading some EPUB3 ebooks to Google Play books. Sometimes the upload seems to take a long time, then stops and gives an error message "Processing failed".
My most recent purchase from Kobo which fails shows "EPUB 3 (DRM-Free)" under the Download options at Kobo. I imported the book to Calibre, which reports the file size as 2.3 MB, well under the size of other boxed sets that have uploaded correctly.
Apart from a bad connection (so that the upload times out), what might be causing these uploads to fail? Could they be malformed in some way that is causing Google Play books to reject them?
Would it help to run the epubs through a validator, or polish them in Calibre? What features might be causing the processing to fail?
I have tried to find an answer via Google support, but the answers I have found so far refer the user to articles about how to download epubs from Google Play, which don't address the issue.