Discovery and Transmission are different processes in making NDI work, so what you are describing is very possible.

I'm guessing that all of the NDI devices are contained on the same LAN. Likely you have a firewall blocking traffic. If these are Windows systems, make sure the network type in Windows is set to Private (not public). If you are using a 3rd party firewall, trying to disable it and see if that gets it working. If it does, then you need to open up in the firewall to let the NDI traffic through.

Here are ports used by NDI.
https://docs.ndi.video/all/developing-with-ndi/sdk/port-numbers