Have a look at your signal strength. It�s usually in your settings menu somewhere. If it�s low you might need to upgrade your cable (to digital quality or better still, satellite � B&Q) from your aerial to your telly. You could start by replacing the bit between the socket and the digibox, that�s sometimes enough.
I sometimes find my box loses a channel and then finds it again. Do a manual rescan when your signal is at its highest (it does fluctuate during the day) to see if this helps the box find your missing channels. But you�re right, the ones you are missing should be there.