You can retune your device to make sure your channels are up to date.
To re-scan for channels, go to settings on your device > general > TV channels > re-scan. This should update your channels.
If you are still missing channels, make sure you have your aerial connected to your device. Get in touch with either the Confederation of Aerial Industries (CAI) or Get Me Digital, who can give you details of one or more members who operate in your area and guarantee their work.
You can find an approved CAI installer at Get Me Viewing or find a digital professional at https://www.getmedigital.com. A local installer will understand the circumstances and local geography in your area and suggest the best option for you. https://www.getmeviewing.org.uk