Please check for the possibility that that the signal may be too strong, and overloading the YouView box.
See these posts.
Too much signal can cause odd signal level readings.
Signal overload is quite common if you are near a transmitter, or have an aerial amplifier.
If the signal is too strong, the digital tuners in the YouView box cannot extract the data from the muxes, they are not all the same strength, which is why some channels work, and others do not.
Try a 6dB attenuator in the aerial input to the YouView box.
They are a very simple inline component, with a TV plug and socket, and fit in the lead, where it plugs into the YouView box.
You should aim for a signal level of about 70-90% with a signal quality of 100%.
See this thread https://community.bt.com/t5/YouView-Boxes/Missing-Freeview-Channels/m-p/1769254#M39534