Occasionally on my XR42A90K I have noticed that the HD channels go AWOL.
A retune usually sorts that out.
Have you checked the the aerial cable is fitted firmly to the TV, sometimes even a little bit of movement over time can result in the loss of signal from one of the multiplexes. Also make sure the solid copper wire is protruding through the hole by a millimetre or so.
As you have retuned several times without success, have you restarted the TV before retuning or tried a power cycle before retuning.
If that doesn't help matters have you tried a manual channel search.
First thing to do is find out which transmitter you are receiving from.
Enter your details here,
https://www.freeview.co.uk/corporate/detailed-transmitter-information
Scroll down you will see which transmitters broadcast to your location.
The first one shown is the 'most likely' take a note of the multiplex number under the BBC B
section, this is the multiplex (mux) which carries the HD channels. The mux number is under the white N in a black circle.
Now you have the HD mux number you can try a manual search using that number to see if you get a signal.
