Snippet from Knowledge base article we use to sort out our problem.

Manufacturers (Intel and AMD, specifically) continually improve their CPUs, releasing new features that may or may not be compatible with VMotion.

To circumvent VMotion’s CPU-compatibility checking for a specific feature or extended feature, you can modify the default bit mask. The bit mask-modification process (sometimes referred to in various VMware documents as “relaxing” a particular constraint) varies according to VirtualCenter Server version and ESX version.

This article provides overriding VMotion CPU compatibility restrictions between ESX Servers.

VMotion CPU Compatibility – Migrations Prevented Due to CPU Mismatch – How to Override Masks

Please do not take this post as technical advise, what has worked for us does not mean it will work for you; please be careful when changing these settings.