Why can't I update to Windows 8.1 after updating to windows 8.1, then resetting it to windows 8?

[WPCentral Question]

I bought a Dell XPS Duo 12" laptop with windows 8 preinstalled. As the 8.1 update came out, I updated and everything was working fine. Then a few pop ups and adverts came and really slowed down my computer so I thought of resetting my whole computer to its factory settings assuming I will be able to update to windows 8.1 later again. However, after resetting and installing the necessary updates for windows 8.1, I could not find it in the store. I then searched for it for a few minutes and click the 'learn more' from a windows 8.1 app that could be installed on the store. It led me to the updating windows 8.1 page. However this time it said 'Sorry, this update isn't available for your PC, or you might be running an edition of windows that doesn't support updating from the Store.' I tried everything from such as activating my computer and downloading different tools and troubleshooters. However later on as I searched, I found that my OS had turned to Windows 8 volume liense, meaning I cannot get the windws 8.1 update. How is this possible? I had a normal windows 8 license and also updated to 8.1 but after resetting, the license had changed and I could not update to windows 8.1.