There's engineering work taking place on the relay transmitters at Humshaugh, Staithes and Falstone. If you get your signal from one of those transmitters it's simply a matter of waiting until the work is completed.
Otherwise you need to boost your signal (using a TV amplifier, for around a tenner, from most big supermarkets). That's because all aerials are better at receiving some channels than others (which is why you've only lost some channels but not others) and the signal strength at your aerial doesn't remain constant. (It's affected by the 11-year sunspot cycle, individual solar flares, atmospheric pressure, precipitation, interference from nearby electrical devices, etc, etc).