Do different groups of channels get broadcast at different strengths, so that although the aerial appears to pointing at the transmitter something that has been mentioned is in effect blocking those missing channels?
Or that if the aerial is slightly off direction only the stronger channels are viewable?
Freeview is carried on a number of multiplexes - typically about 5 for standard definition, and another one for high def. Each multiplex occupies one "channel", the same width of radio spectrum that used to carry just one analogue channel. Various parts of the system will have slightly different handling for the different frequencies - the transmitter might not be putting out exactly the same amount of signal, the antenna has a gain that varies with frequency, and the TV tuners will have slightly different sensitivities to different frequencies. Add to that, if there is noise about, that will vary with frequency as well. So all in, it's possible to be able to receive (just) a subset of the muxes, but not the others.
I find the notion that a channel can be lost if the signal is too strong difficult to understand too!
Any system is designed to handle a certain range of signal levels. Too little and you can't hear - just like you can't hear someone who is whispering and is a long way away.
What people often don't understand is that if you feed in too much signal, it can overwhelm the system. Ever been to some sort of live performance where they've turned <something> up to 12 and what should be nice clear music and singing is turned into an unintelligible noise ? Such distortion on a Freeview mux would be enough to stop the receiver working.
On top of distortion to the signal that's wanted, there are several other frequencies being transmitted nearby (the other muxes), and when you start overloading the electronics, it's not uncommon for adjacent frequencies to start to bleed over - akin to going to the aforementioned even and as well as the sound system being much, there's someone stood next to you shouting through a megaphone.
As an aside, it's been well established that the human ear exhibits a lot of masking. If there's a loud sound present, then the sensitivity to other, quieter, sounds is reduced. Thus if the volume is turned up, you can no longer pick out the detail. This effect is used in MP3 compression - the encoder effectively models the human ear and works out which bits of the original signal you won't be able to hear - and throws them away. The result is a signal that can be encoded into a lesser quantity of bits.
Similarly, wearing ear protection doesn't just help avoid you going deaf - it actually allows you to hear more. It's counter intuitive, and I've failed to persuade a number of farmers, but wearing ear protection can actually make it easier to hear other sounds (such as a new noise from a piece of machinery that's starting to fail).
