Phone notifications

I have an iPhone 11 Pro max and a few days ago it stopped giving me the notification when only one person text me. It will notify me for everyone else, but not for the one person. I saw where I could delete the messages and start over but I do not want to lose my messages. Its like whenever she text me, I don't get the notification but ill open my phone and the message will just be opened like I opened it and left them on read