Not sure if this has been covered before but was helping out some relatives with a YouView setup and found that even though it worked totally well with their previous Humax PVRs and Digital TV, the existing aerial flylead was the culprit. Once this was replaced with what was claimed to be a shielded lead all the channels they had before appeared and in improved quality as well.
It seems that the BT YouView tuners are a bit more fussy than previous tuners. I had initially thought that they had a duff box.
I replaced the lead with one of these 3m Coax Cable - High Quality - 24k Gold Plated Plugs from amazon: