In my place of work we had a 55 inch tv on the wall. This is fed via an HDMI cable that snakes its way across the ceiling void rom a CCTV recorder and displays a grid of CCTV images. Workshop, stores, front door, that sort of thing.
Because reasons the 55 inch TV has been replaced with a larger TV, repurposed from elsewhere. Plug it in, no picture. Turn the CCTV recorder off and on again, no joy. Plug lap top into larger TV, nice picture of my Desktop. Connect another, smaller, monitor to the HDMI lead normally feeding the big TV and we get a picture. Plug the smaller monitor directly into the CCTV recorder and we get a picture. So the big TV works and the CCTV recorder is providing a signal.
Yes, the big TV input (HDMI n) matches the number of the socket the cable goes in to.
What obvious thing am I missing?