There are TWO reasons why you might be missing some Freeview channels:
1. (The main reason, and the one which applies to your problem with Quest)): Freeview channels are transmitted in 'bundles', called 'multiplexes', with each multiplex being on a different frequency. Theoretically the lengths of the elements on your TV aerial need to exactly match the frequency that the aerial is designed to pick up so, in an ideal world, you'd have a different aerial for each multiplex. (i.e. you'd have loads of aerials on your roof). Clearly that's impractical, so we use aerials which are only a 'rough match' to the various frequencies used by different multiplexes. That means that any aerial will be better at receiving some multiplexes than others. So, if the signal strength from the transmitter isn't good enough, your aerial will be able to receive the channels on the 'well-matched' frequencies but not those which are on 'poorly-matched' ones.
The solution is simply to fit an aerial booster. They're sold in some larger supermarkets for around a tenner. (Remember to retune your Freeview box after fitting the booster)
2. If you've only got an SD ('standard definition') Freeview box you need to be aware that there are a few channels which, although they only broadcast in SD, are forced to use HD multiplexes due to lack of channel space. So channels such as Talking Pictures, Vintage TV, Viva and Rishtey (all of which transmit only in SD) can't be received via an SD Freeview box; you need an HD box to get them