Theoretically, an aerial should have the length of its elements exactly matched to the frequency of the station it's designed to receive. So, in a perfect world, if you wanted to receive 30 different TV channels, you'd need 30 aerials on your roof and you'd have to change aerials every time you changed channel.
Obviously, that's not possible, so any TV aerial has to provide a compromise solution. (i.e. it's roughly tuned to all of channels but some will be better matched than others). So it's inevitable that, if the aerial is barely good enough, some channels will have good reception while other won't.
It's also been stated, in previous posts here on AB, that the ITV channels use a different form of digital encoding to that of the BBC ones, which requires a more efficient aerial. (I've not seen any technical information to substantiate that claim but, in view of the fact that most people lose their ITV channels before the BBC ones, it would seem to make sense).
Many digital transmitters are currently running on reduced power, so as not to cause interference to analogue transmissions. When your area finally goes completely digital (i.e. when the analogue transmitters are switched off), the signal quality will probably improve as the transmitters will then run at their full power. If you can't wait that long, you should upgrade your aerial system. The most frequently recommended aerial, here on AB, is this one:
http://www.maplin.co.uk/Search.aspx?criteria=p t18u&source=15&SD=Y
If you've already got that type of aerial, check out the links for 71, 83 and 91-element aerials at the left side of that page. Whichever aerial you go for, remember that it's important to upgrade the aerial cable at the same time.
Chris