I only have a very basic understanding of ARD when I used it many years ago. I remember having to individually scan multiple subnets in order to see all the Macs I have. I believe there was a way to specifiy your subnet ranges in the software. Have you tried setting up multiple scans?
I'm not sure if it attempts to use Bonjour or multicast DNS to dynamically find devices or if it's just a simple subnet/port scan.