There's one number (of a relative) I text regularly who doesn't always get them. It is just this one number. If I phone, their mobile always goes straight to voicemail. Anyone else had this and what sorted it out? I have googled and forcing your messaging app to stop briefly was one option. I have lots of texts on mine which I do not wish to lose (but my phone has enough memory to cope.) They do not have a smartphone, btw.
If they have a really basic non-smart phone, it's possible that they've run out of storage, which means text messages may not be received. I had this with an elderly relative of mine. They needed to delete texts they no longer needed to keep, including Sent messages, in order to receive new ones.
The phone they have is odd in that it seems to have barely any storage at all, but I think only the received texts are being deleted. I'd best ask.
Some older basic phones will only store a limited number of text messages.
Here's the thing though. Newish phone, rubbish storage. Same with the one before that.