Firstly, do a re-tune on the box.
There will be a menu option to do a re-scan somewhere.
The channels have moved around a fair bit over the years, and your box has no way of knowing that till you force a re-tune.
Yes, the channels are multiplexed.
So several channels share a single 'mux', which is broadcast on a single UHF channel.
There will be several muxes broadcast from the transmitter, each carrying several TV channels.
The act of 'tuning' a digibox basically involves it scanning through each of the UHF channels, looking for a digital mux.
Then it interrogates that mux for what TV or radio channels it contains.
It then builds up a table of channels to display to you, and when you select one of the TV channels, it knows what mux to go to and which stream in the mux to decode.
If the box has not been re-tuned in many years, the table will be out-of-date, and when you select some channels it looks in the wrong place because it's moved.
If that doesn't help, then the digibox is probably past it's use-by date.
The terrestrial digital TV system has evolved, and many of the earlier boxes were not particularly compliant with the spec.
One thing that changed was the NIT ( Network Information Table ) got split some years back.
This was always in the spec, but many original boxes didn't support it because they didn't need to at the time.
Then HD got added.