To {try} to put this into layman's terms;
Imagine that details of each of the available channels is stored in a list inside the machine (called the NIT). This has become bigger as more channels are launched and, an upgrade to some transmitters (which will occur nationwide) means that there is now more info being broadcast about each channel - which must fit into the list.
Before the "rules" were fixed about how much space (memory) to have inside the machine to hold all this info, older design boxes only had enough space to store the information about the channels available when they were made and no spare space for new channels. They have now run out of space to keep all this extra info. This means that if you do a re-scan (or re-tune) of the channels they either don't find some channels, or don't recognise them.
The quick answer is NOT to do a rescan, as this tries to make a new list with all the new info and then the box fails. If you leave well alone then these boxes will still work, albeit maybe missing some of the newer channels. If you get a message on-screen saying "New channels are availabe, do a re-scan" always click NO, for now, until either your box maker issues new operating software or confirms it will never be fixed for your box.