First, check your cables are connected correctly. Especially the cable from your aerial to your BT TV box.
Next, check the TV signal and quality for the missing channels. To do this will depend on which type of BT box you have:
If the signal quality is less than 50%, this will indicate that you have a weak signal to receive this channel. If you're signal is too high this could also cause the issue. You could try connecting your BT TV box to another aerial port in your home if you have one to see if this makes a difference.
If you have a good signal strength on all the other channels (greater than 80%) then try re-tuning your box and scanning for channels:
Finally, if you have: