Is this with the same aerial ? Do you have any splitters or amplifiers connected to the aerial ? Is the aerial external ?
Can you try removing the aerial from the youview box and try a retune. Connect the aerial back to the Youview box and then retune again and see if any channels are picked up ?
It could be possible, but I've not seen that before without an amp or signal booster in place. you should still see something.
We can at least verify the tuner has not failed by trying another.