In this situation, there are several things you can do. First, try logging out of the app/browser service and logging back in again.
If the book still fails to appear in the app, then refresh your bookshelf, by ensuring you have an internet connection and pushing the refresh button in Android or pulling down the page in iOS.
If it still fails to appear, or if you are using the browser, then try refreshing your bookshelf using the designated page refresh button in your browser.
If the ebook still does not appear, please contact Customer Services.