Issues with Authorising Single Use Features on ArcMap 10.2.1

Does anyone have any ideas why a user can not authorise single use features but can use Concurrent Use Licenses successfully.

User has authorised the ESU on a different desktop and it works, same versions running etc..

Trusted storage has been cleared out,
Have tried authorising via internet and through Esri website
still no luck. ( I did deauthoirse the ESU on the other desktop)
The correct license level (Single Use) was selected in the ArcGIS administrator and the key is for the correct version.


Any ideas is appreciated
Thanks
John