There are three possible reasons for this. Either it is a caching issue within the app (this only effects Android users), or it is an account issue, or it is a package issue.

If you are using an Android device please log out of your account within the app.
Then close the app and go to 'Settings' within your operating system.
Go to the 'Application Manager' and select the 'Ebook Reader' app.
Now choose to 'Force stop' the app.
Then 'Clear cache'. (Please note DO NOT 'Clear data', as this will delete all your books from your device and you will need to redownload).
Once you have finished these instructions re-open the app and log back in using your eBooks.com account details and the books should sync.

The other likely issue is that you have not correctly made a purchase, so the book is not stored against the account. The alternative is that you have purchased the book on one account and logged in with another account. For example the purchase was made using mmyname@example.com, and you are logging in with myname@example.com

In these instances please contact our support team. Please ensure you quote the invoice number of your purchase so we can find it regardless of which account was used to make the purchase.

Alternatively, you may receive this error message if you have bought an ebook package (more than one ebook in a single product). For more information see the help article here.