Confirm you have the small notification icon on you taskbar?
If not, you may need to recheck for updates and make sure you have those required most important one is KB3035583,but you may also see KB2990214, and KB2952664.
Presumably your 8.1 installation is activated?
You can check this by right clicking "This PC" and selecting the properties.
When you reserved your upgrade, did you supply your Email address to get the confirmation?
If these points have been checked, perhaps your only option is to go through the reserve process again.
You must allow the upgrade to proceed, in order to have it registered on your machine. Subsequent fresh installs will not, after that, require a key at any stage. It is Microsoft's new licencing method.