Hi @Anonymous User
To double check your TV aerial can you plug the TV aerial directly into your television (assuming your TV has an inbuilt Freeview tuner) and rescan the TV channels on your television.
This might give you an idea where the problem lies.
Also i am assuming you are not splitting the incoming TV aerial to other devices in the home.
Also i would be tempted to do another factory reset by going into the NowTV box settings > Systems > Advanced System Settings > Factory Reset.
Once factory reset the box, try a fresh rescan on the NowTV box again (don't forget to reconnect the TV aerial).