I just had a similar problem last week while trying to send NDI and OSC to and from a powerful Windows show computer that belongs to a university that has very strict network and computer security protocols. I never did figure out what setting on their computer was preventing it, but when I hooked up two non-university computers to a network switch I was able to send NDI and OSC back and forth without issue. I suspect it was a firewall problem, but nobody around had sufficient security credentials to do anything to the firewall's policies regarding Isadora on the university computer. So maybe check your firewall settings on both computers? Also, as a test, I'd hook up a third computer to your network switch and see if it can send and receive NDI to and from your original two computers. This test might help you identify which one of them is the problem.