Sometimes when I return it doesn't even find the HD channels
Then the poor signal quality is being introduced earlier on in the chain. Take the attenuator out, and insert the 6dB attenuator on the input of the amplifier, between the aerial and amplifier input, that will prove whether its the amplifier overloading.
I would still suspect that the amplifier, intended for analogue signals, is not coping well with the digital ones, or its suffering from 4G interference which is degrading the signal quality. 4G may have only recently been enabled in your area.
What I cannot understand is, when you connect the aerial directly down the lead to the YouView box and TV, you get no signal at all on either, unless your aerial is badly out of alignment, or faulty
Connect the black cable from the aerial outside, via the female adapter, to the lead that goes to the YouView box downstairs, which I think you said you identified as being connected to the UHD out.
You will lose signal to all of the other TVs, but its only to eliminate the amplifier as a possible cause.
See what signal level and quality is showing on the YouView box (remember to remove the variable attenuator)