Add VM to Virtual Network

I have created several VMs as part of a virtual network. Any new machines I create, however, are unable to attach to the virtual network - either using PowerShell or UI (on creation).

Looking at the new VM properties in PowerShell, the 'VirtualNetworkName' is blank and I receive the following error when I try to move the VM to my virtual network using PowerShell:

"Update-AzureVM : BadRequest: The virtual network ID cannot be null or empty."

The only thing I can think is that the original VMs and virtual network were created under another subscription and moved to the current subscription.

Any help appreciated.

Kind regards,

Lewis