Hello. My name is Michael. I can help you with your question.
The first thing you will have to check is to make sure that you have used the same account to purchase the book as the account that you have used to set up your reader. The account on the reader determines what is downloaded to it.
If that is checked and is OK, then how you proceed will depend on the model of your reader and the retailer from whom you purchased the ebook. If you can provide that information, I can provide you some additional specific info.