Theoretically the lengths of the elements of your aerial should be exactly matched to the frequency of the incoming signal. Since every channel is on a different frequency, you should ideally have a different aerial for every channel which you watch. That's clearly impractical, so we use TV aerials which provide a 'rough match' to each of the different frequencies. That inevitably means that all aerials will be better at receiving some channels than others. Your particular aerial is weakest at receiving C4.
The good news is that you probably don't need to buy a better aerial. Your existing aerial is almost good enough for the job, so a cheap signal booster should fix the problem
Chris