For some reason (hope to resolve), the Google Books search API used by RetroRead does not always return the same results as a user searching directly on books.google.com — with the result that a user finds a book on Google, but can’t convert.
There is a “backdoor” url:
That allows you to manually upload a file from your desktop (you need
to be logged in). So if you find a file on books.google.com, download it to your desktop, log in, then go the /gbif/ url and in the header there is a file upload button; click the button, find the file on your desktop, and the file will upload and convert, and be delivered per your configured delivery settings.