I have been having the same problem with only the BBC channels for the last couple of weeks.
Freeview sent me the following info which might help: (check your transmitter for problems first).
The channels are broadcast in six groups, with each group on a different frequency, which is why you can get problems with a particular channel group, while other groups are unaffected.
This is most likely an aerial problem of some kind, whether with the
antenna itself, the cabling or connections.
We would recommend following this procedure to clear the memory of your
digital equipment and then rescan for the channels:
Remove the aerial from the equipment and leave unplugged.
Go to the set up menu and select rescan or retune to replace existing
channels. It’ll take a few minutes, but when it’s done, it should
say ‘'no channels found’.
Disconnect the equipment from the mains power supply and leave it
un-plugged for 30secs.
Re-connect both the aerial and the mains power supply and switch the
equipment back on.
Go to menu again and select rescan or retune. You should now have all
of the channels.
If this procedure does not resolve the problem, we would recommend that
you get in touch with the Confederation of Aerial Industries (CAI)
www.cai.org.uk or the Registered Digital Installers (RDI)
http://www.rdi-lb.tv/installers.html who can give you details of one
or more members who operate in your area and guarantee their work.
Sadly I cannot do all these checks myself as I am not in the UK and viewing the Freeview box via internet streaming. I have rescanned all the channels which I can do remotely but was not able to follow the instructions exactly.
Would be interested to know if this works for anyone else having this problem. It is definitely only BBC channels that I have a problem with.