You can do a single frequency scan and the chances are this will pick up chanels that have not been picked up on an automatic full scan.
Then do the following:-
Press menu;
Use arrow keys go to installation (3rd one along);
Go to installation;
User settings enter PIN 0000 click on ok;
You will see a list of 40 or so frequencies channels;
Scroll down to each one that has the frequencies
Now press ok and you will see a tick appear in the box of the channel number - you could do this for more than one frequency if channels are missing;
Press the green button "manual scan";
This scans the channels and re-installs them in a matter of seconds;
New channels will be found at the end of your tv list.
If you can get a copy of a cdf file for your area flash that to your box and you should get all channels.