Can you unplug the aerial from the youview box, connect it directly to the TV and retune and see if all channels show on the TV ?
This will help to check if it is an aerial issue.
so can I confirm, when the aerial is plugged into the TV directly there are no channels showing ?
If this is the case it may indicate an aerial issue rather than a box issue.
Hope I've interpreted that correctly.
If your TV has freeview, when the aerial is connected directly to the TV and not to the youview box, then the TV should pick up freeview channels.
If the aerial is going directly to the TV and not the box and no channels are being picked up, then this could be the aerial itself or the connections that are at fault.