If it works with your other Kobos it's strange for this one to fail. If it didn't even see the network in the first place, it could be a hardware issue, but...
Did you factory reset it or just format the user partition? It keeps a wifi config file in the root filesystem that does not go away until you factory reset so it could be some leftover getting in the way.
If all else fails you'll just have to setup through PC, update it to latest firmware (download links in this forum) and hope it will work then...