NEW Apple TV and Smart TV

My Apple TV is connected through the HDMI port. I have previously connected to Apple TV by selecting the HDMI-2 (at the time) from the tv’s input selector with no troubles. I have watched it several times. Now every so often it doesn’t find the Apple TV - I hadn’t change the port or anything. Then o e day it worked again with no changes made.

move tried using a different port, that had previously connected another device successfully, but now that won’t find the Apple TV either. The same pop up message comes up as did when I used the other HDMI port - “weak or no signal”

I have no idea why some days it works and others it doesn’t ..?