Missing channels are usually a sign of poor signal strength. (Digital TV channels are transmitted in 'bundles', called 'multiplexes', with each of those bundles using a different frequency. All TV aerials are, by the laws of physics, better at receiving some frequencies than others so, if the signal strength isn't good enough, you'll lose the channels within particular 'bundles').
Missing data (e.g. that required to update your EPG) is another indication of poor signal strength. (The signal strength at your aerial isn't constant; it's affected by the weather, atmospheric pressure, the solar sunspot cycle, interference from electrical devices used by your neighbours, trees blocking the signal path, etc).
So the first thing to do is to check that your aerial is properly plugged into your Humax recorder (and also to check any other connections in the lead). Then take a look outside to check that the wind hasn't knocked your aerial out of alignment. Also zoom in on this map to check that there's not any temporary engineering work reducing the signal strength in your area:
http://www.ukfree.tv/maps/live
If you can't locate any problems there, a simple signal booster might well fix your problem. All of the big supermarkets sell them (usually for around a tenner) but this one from Maplin, although rather more expensive, offers greater 'gain'. (It's what I use myself).
Given that you already get the majority of channels anyway, that's likely to solve your problem. Otherwise you'll need to get an aerial fitter to upgrade your aerial for you. (NB: The
majority of aerial installers probably know very little about their job. Make sure you find someone who actually does, via this website:
http://www.cai.org.uk/members/member-search )