Solution 1:

The 8.1 Update is only shown to Retail licenses and not Volume Licenses (volume_kmsclient) of Windows 8.

Why can't I find the update in the Store?

http://windows.microsoft.com/en-us/windows-8/why-can-t-find-update-store

Some editions of Windows 8 don’t support the free update to Windows 8.1. These include:

  • Windows 8 Enterprise
  • Editions of Windows 8 Pro that are installed by enterprises using KMS activation
  • Editions of Windows 8 that are installed using an MSDN ISO, and activated using multiple activation keys