Hyper-V no classifying VM

Hyper-V no classifying VM

I have a Hyper-V host but it is not classifying some of the VMs as a 'Virtual Device'. I am unable to change the category because I get an error which says:
 
'The Device Cattegory cannot be changed to Virtual Device, if not discovered as a virtual device'
 
Does anyone know why this may be the case?

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