All aerials are a 'compromise'. Theoretically, for maximum signal strength, you need a a different aerial, precisely matched to the correct frequency, for every single channel. Obviously, that's not possible, so any aerial is going to better at receiving some channels than others.
It seems that your aerial isn't up to the job when it comes to 'pulling in' the signals which it's least well matched to. The fact that this has been the situation for the past three weeks could be due to weather or atmospheric conditions (for example, such things as solar flares) but, if it's been continuous, it's far more likely to be a physical problem with the aerial. The most obvious one is that the aerial may no longer be perfectly aligned with the transmitter, after being battered by the wind. Alternatively, there could be water ingress into the connections between the aerial and the cable.
Either way, it's time for somebody to climb a ladder.
Chris