NTM mapping

Unless I use a seed device only the main device comes up as a discovered switch with a switch icon.  Every other switch, router and other devices come up as undiscovered and do not display with a router or switch icon.  What stops NTM from reading the vlans on a switch, thus prohibiting it from being a seed device?  Also why after adding the vlans as either an ip range, subnet, etc does it not discover the devices.