Digital terrestrial TV has experienced lots of problems lately (probably due to sunspots). The channels one person has difficulties with may not be the same as his neighbour. A perfectly designed aerial for Channel 5 would have slightly different dimensions from one which was perfect for, say, BBC1. (This is because different frequencies require different dimensions for the aerials). Obviously, we can't have a different aerial for every channel we require so aerials are manufactured on a 'one-size-fits-all' basis - but they're not all identical. Your aerial might be excellent at picking up BBC1 but poor for Channel 5. Your neighbour could have an aerial with the opposite properties. So, when reception conditions are poor, you'll have problems with C5 - he'll have problems with BBC1. (The experts out there will point out that things aren't actually quite as simple as this with digital TV but the basic principles still apply).
Hoping this helps,
Chris