Xbox won't display on 720p but will on 1080p TV

Hopefully someone can help. I've searched but cant find the solution. I have an Xbox One that was hooked up to a 720p TV. I stopped displaying recently when I disconnected it. I plugged it into a TV that is 1080p and it works just fine. I've plugged
it into a 2nd 720p TV and it won't display. All TVs work, all ports work, all HDMI cables work. I've changed the display settings on the Xbox and it doesn't matter. Any help?

:)