I had the same problem too - on the both top and bottom floor of my house i wasnt recieving a good signal at all even when using an amplifier. This is really annoying especialy as my analogue tv and sattelite works fine.
I used to live outside the city of leicester but moved in the middle of the city now. But I used to get reception at my old house whereas i get next to no reception at the house in the city centre.
Its quite odd really. I was advised to have a proffesional check my aerial on the house but i havent done that yet - It may be that the aerial is wideband or isnt specialized for digital tv ( probably a very old one)