If your book is DRM protected (indicated by the small danger sign on the book’s file) and you have already opened it on 6 different devices, you can no longer authorize any additional devices. If you wish to do so, the book must be purchased again.
If your book is not DRM protected, it is however possible, that certain editors limit the number of permitted downloads. If you have already downloaded your book 3 times, it is possible that your license has expired. Send us a request, explain your problem, and be sure to include your order number.