I'd really try setting up Discovery Server and see if that helps. If you are having to use External Sources to get devices appearing, then something is blocking the mDNS protocol from working on your network which is how NDI tries to work by default.

To use Discovery Server, run it on one of your Windows PC by going into "C:\Program Files\NDI\NDI 6 Tools\Discovery Service" and running the executable there. You will see a console window appear saying the server is running on port 5959.

Make sure no NDI application are currently running or open on the computer.  Then open NDI Access Manager. In the Advanced tab, enable Discovery Server and enter the IP address. You must do this on all of the systems.

I'd also remove the External Sources you have manually entered, just so you are only testing with Discovery Server.

Once you have this setup, when you start an NDI application you should see an entry appear in the Discovery Server console display.  See if things work correctly.