just had another one of those.
auto discovery was blank on any level, in other words no subnets or nodes discovered
added nodes under node discovery, all of them, and looked at the event file
event file said for all the nodes, check seed file
deleted all the node discovery entries, went to network discovery and disovered NOT the whole subnet but set of nodes starting from 172.20.1.1 to 172.20.1.50. Also unticked the discover local subnet box.
ALL 6 of the controllers have been auto disovered.
Hope this helps in a way.
Thanks
Ps like I've said, I've had to do this to probably 6 installs already and without fail this method has worked. good luck!