Yes this does happen and can be for a variety of reasons such as:
The book isn't released for another few months and our system hasn't picked it up yet.
The book is listed but has an alternate title.
The book exists but isn't available for purchase anywhere and our auditor missed it when collating the author's data, or they just simply missed it.
Simply contact us to let us know. Our mailbox is monitored all day long and if there is a missing book, we'll have it added within 24 hours. We appreciate any help in making our lists as complete as possible.