It's fine for me.
If you're losing certain channels it's usually a sign of poor signal strength. (The signals at your aerial aren't consistent in strength. They're affected by all sorts of things, including atmospheric pressure, precipitation, solar radiation from flares on the sun's surface, interference from electrical equipment in your home and those of your neighbours, etc). Different groups of channels use different frequencies, with TV aerials being better at receiving some frequencies than others. So when receiving conditions are poor, if you're aerial's not fully up to the job you'll lose reception on certain groups of channels. (If you're having problems receiving Ch 71, it's likely that you'll have problems with Channels 19, 31 and 79 - among others - because they share the same 'multiplex').
So you need to improve the strength of the signal getting to your BT TV box. A simple signal booster, for around a tenner from major supermarkets, together with an aerial fly lead (for a pound or two from 'discount' stores) should solve your problem.