You can retune your device to check to make sure your channels are up to date, to re-scan, go to setting on your device > general > TV channels > re-scan. This should update your channels. Make sure you have your aerial connected to your device
If you are still missing channels,
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:
Find an approved CAI installer at Get Me Viewing.
Find a digital professional at getmedigital.com.
A local installer will understand the circumstances and local geography in your area and suggest the best option for you.