Consider the following scenario:
You have a Hyper-V host that is running x64-based versions of Windows 8.1 or Windows Server 2012 R2.
You create RemoteFX feature enabled virtual machines (VMs).
You turn off these VMs.
In this scenario, the virtual RAM (VRAM) is not released. Eventually, these VMs cannot start because of insufficient VRAM.
To resolve this issue, install the November 2014 update rollup for Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
See the terminology that Microsoft uses to describe software updates.