Ideally, the lengths of the elements on a TV aerial should be exactly matched to the frequency of the incoming signal. Since every channel is on a different frequency, you theoretically need a different aerial for every channel you watch. Obviously that's not practical, so all TV aerials have elements with lengths which are only a 'rough match' to each of the channels its designed to receive. That means that any particular aerial is likely to be better at receiving some channels than others.
Your aerial seems to be weakest at receiving Channel 4. However, because the signal strength is usually strong enough, that's not normally a problem. But if the incoming signal strengths are reduced by some form of interference, then it will be Channel 4 which you'll lose first.
As Hopkirk mentions, there's a high probability that your current problem is related to solar activity:
http://www.bbc.co.uk/...ld-us-canada-12516918
Chris