This is a classic symptom of insufficient signal strength..
Theoretically, an aerial needs have elements which are exactly the right length for it to 'resonate' with the frequency of the signal it's picking up. So, in an ideal world, you'd have a different aerial for every single station. Obviously, that's impractical, so any aerial will be better at receiving some frequencies than others. Yours is obviously not very good at receiving the ITV and C4 transmissions. (I've seen another post on AB which suggests that the ITV signals are also encoded slightly differently, which makes them harder to decode with a weak signal. I'm not sure whether that's correct but it's nearly always the ITV channels which 'drop out' first when the aerial isn't good enough).
You might need to upgrade your aerial system. ('System', here, means the aerial and the cable). However, because your signal is nearly strong enough already, you might be able to get sufficient improvement by just using an aerial amplifier. (I've not looked recently but Asda and Tesco Extra used to sell them cheaply).
Chris