some texts not being delivered

Hi, as above, some, but not all texts are being delivered. It’s not been for very long but can’t quite remember if it happened before I installed the latest update. It’s VERY annoying and one text that wasn’t delivered was VERY important; I can’t trust my texts to go through. I’m NOT getting a failed text notification. I don’t text very many people and so far as I know, it’s been only 2 diff numbers. One person has recently got a new phone the other hasn’t. My mum got a text this morning that I sent over a week ago. I am trying to find solutions with my network provider, but could it be something with my settings? It’s SO very annoying, and inconvenient! Thanks xx