You almost certainly need to upgrade your aerial system.
In an ideal world, the length of the elements of an aerial should be exactly matched to the incoming frequency, to provide perfect 'resonation'. It's possible to do, but only if you have a different aerial for every channel that you watch. (So you'd need dozens of aerials on your roof). That's clearly impractical, so all aerials can only offer a 'compromise solution'. They'll be very well matched to some channels, but less well-matched to others.
When digital television wa first planned, it was estimated that one third of all homes would need to upgrade their aerial systems. It seems that the actual figure is rather lower but it should hardly come as a surprise if your own system is inadequate.
You need to improve your aerial system by installing a new aerial
and new cabling. (Older cabling suffers far greater signal losses than modern co-ax, which has been designed specifically with digital TV in mind).
This aerial has been widely recommended, and praised, by contributors to AB:
http://www.maplin.co.uk/Module.aspx?ModuleNo=2 2523
Chris