Problem attaching USB ports (drives) to VM

I am running KVM on Ubuntu 18.04 Desktop. The physical machine has eight USB ports, of which I have three connected to drives.

I'm having trouble connecting the USB drives to a VM using libvirt/virt-manager. When I try to add an additional USB through "Add Hardware" in the virt-manager GUI, I get a message saying "Error adding device: Internal error: No free USB ports". But, I am not even close to using all the USB ports of the physical machine, so I'm not sure why additional ports can't be added.

Where am I going wrong? What is the correct way to set this up?

Thanks
Eric