'Missing channel' problems usually come down to an aerial which isn't fully up to the job.
Quest is part of the Arqiva B 'multiplex' (= 'bundle of channels'), whereas Quest+1 is within the Arqiva A multiplex. Each multiplex uses a different frequency and it seems likely that your aerial doesn't 'resonate' as well with the Arqiva B frequency as it does with Arqiva A one. (You might also experience some problems with other Arqiva B channels, such as W, Yesterday and GREAT! Movies).
The solution is to fit one of these
https://www.argos.co.uk/product/9383027
for which you'll also need one of these
https://www.screwfix.com/p/philex-coaxial-cable-1m/41720
and then to re-tune.