Limited reception on some channels?

I have recently bought a (v) cheap Alba DV3 Set Top Box and have great reception on some channels (BBC Channels, Sky Sports News, Channel 5 sometimes). However, the reception for ITV and Channel 4 (and all sub channels of these - E4, More 4, Film 4, ITV2/3 etc) are very patchy, frequently just giving sound and often v jumpy images. Any ideas why this might be, or what I can do? The signal strength displayed is the same for all channels, so the limited pictures seem surprising.