I've provided the following answer well over a hundred times here in the past and, to the best of my knowledge, it's never failed to resolve the problem yet!
You've described the classic signs of insufficient signal strength. Freeview channels are transmitted in 'bundles', called 'multiplexes', with each multiplex using a different frequency. Theoretically you should have a different TV aerial for each multiplex, so that the elements can be exactly the right length to 'resonate' with the relevant frequency. That's impractical, so we all use aerial which are only a 'rough match' to the frequencies in use. That means that any aerial will be better at 'pulling in' some multiplexes than others.
The BBC channels are all transmitted using the 'PSB1' multiplex and that, it seems, uses a frequency which your aerial is only poorly matched to. So, if reception conditions aren't at their best, you'll lose the BBC channels.
Reception conditions don't remain constant. They're affected by many things, such as the 11-year sunspot cycle, individual flares on the Sun's surface, atmospheric pressure, precipitation, transmission path obstructions (such as new building being erected between the transmitter and your aerial or even just a tree growing), interference from within your own home (e.g. from a computer or inkjet printer), interference from neighbouring properties, etc, etc.
Further, the introduction of 5G mobile phone signals has forced some changes to the frequencies used by Freeview transmitters and, importantly, has often required the use of lower power by those transmitters.
So, for whatever reason, your aerial isn't up to the job. (When we switched over from analogue to digital TV it was estimated that around a third of households would need to get new aerials. That didn't happen but it should hardly be seen as surprising if millions of people are still using aerials that aren't as good as they could be at 'pulling in' signals when reception conditions aren't at their best).
The vast majority of people who encounter the problems that you're now experiencing don't actually need a new aerial though. It's usually possible to improve the signal strength sufficiently simply by purchasing one of these
https://www.argos.co.uk/product/9383027
and one of these
https://www.screwfix.com/p/fly-lead-coaxial-plug-to-plug-1m/41720
(I recommend retuning your set after fitting them).