Two possible reasons come to mind....
1) Aerials are tuned to receive signals centred around a certain frequency and yours is tuned to receive the frequencies the BBC channels are transmitted on but it is rubbish at receiving the frequencies the other channels are on, solution... an aerial upgrade.
2) some areas that have yet to fully switch over are running the digital broadcasts at reduced power for certain frequency bands, and some are reduced in certain directions (ie, my local transmitter runs a reduced poser to the east of it). if this is the case for you then a decent, wide band aerial possibly with a mast head amp my help, but it may not!.
Realistically, without knowing exactly the location your house is in, what the surrounding terrain is like and where your local transmitter is all anyone can do on a website is take a good guess, you need a proper site survey down by a decent aerial installer.