There are all sorts of reasons why groups of TV channels might be missing
A knowledge of how TV Channels are broadcast might help

What is a Mux or Multiplex?

TV Transmission
The free or terrestrial digital channels picked up via a normal TV antenna from a nearby transmitter are transmitted in a tight frequency range band or multiplex or mux

Frequency and Channel Compression
Each frequency range can have several TV channels compressed together for transmission which are then decoded and separated by your TV, or set top box

For example,
BBC
Many of the BBC channels BBC1, BBC2,& BBC3 would be compressed together into a single digital signal band or multiplex to be decoded and separated by your TV or other receiver
ITV
The ITV, Channel 4 & E4 Stations would also be compressed together into a separate multiplex transmitted at a different frequency

I Can’t Get Some Digital Channels

Poor Freeview Coverage
If you can’t receive all the multiplexes from your nearest transmitter it may be that they aren’t covered in your area but there is likely to be a different explanation

Channel Changes
Sometimes certain channels numbers or allocations change and you will need to re-scan to recover the missing channels

Transmitter Failure or Maintenance
When scanning for channels fails to find the missing channels it’s worth checking that the TV transmitter is functioning correctly and is not down or running at reduced signal during essential maintenance

Aerial Alignment
Modern aerials are pretty good at receiving all the multiplex channels; if you originally received all channels it may be that your aerial alignment has changed and one or two of the less optimal multiplex frequencies now has too low a signal strength

Aerial Performance
Aerials need to collect a range of frequencies and if you have just bought a new TV or digital receiver it may be that your old aerial has insufficient gain or bandwidth to differentiate all the multiplex channels and needs updating

Poor Cable Performance
Once your aerial collects the signal it’s passed via an aerial cable to your receiver; poor cables or connectors can degrade an already weak signal and cause weaker signal channels to be dropped

Powerful Signal
It is possible to get too strong a signal from a nearby transmitter or relay which overwhelms the receiver; making it difficult to decode certain channels; if this is the case, an engineer can fit an attenuator between your aerial and reciver to output an optimal signal

Equipment Interference
If you have various pieces of equipment capable of receiving and outputting a signal; try turning off non-essential equipment to see if you can receive or retune the missing channels