Trouble loading e-book on new computer (Surface Pro 3)

I have an ebook that I purchased that is supposed to be able to load on 6 devices. I have it successfully loaded on one device, but I cannot get it to load on a second device. The second device is a Microsoft Surface Pro 3 computer running Windows 10. The other device is a HP Desktop computer that was running Windows 8.1 when I downloaded the file, but is now running Windows 10 and it works OK