Anne's iPhone 8 has been skipping notifications lately. She took it into the Apple Store, and a Genius said to reboot and reset the phone. She seems only to get notified if she's in the Messages app. Leo says that iOS tends to shut down apps you're not using, but Messages get priority. Why it's not notifying you is a mystery. You want to double-check your notification settings to make sure the chime is enabled.
The chatroom suggests that the order of steps is important. Turn off notifications, reboot the phone, then turn notifications back on. Reboot again. That should wipe out the issue and restore your default notification settings. You may also want to uninstall a rival messages app, like WhatsApp, to ensure it's not "capturing" the notifications.
There are also a few things to look at like filtering. You could be filtering out some unknown sender messages. Check out this link.
Anne's friend also has problems remembering passwords, and now his iPhone and desktop is activation locked. Leo says the iPhone has a separate passcode from the Apple ID, as does the computer. So it's no surprise he's forgotten. It's very common. You can take the phone to the Apple Store and they may help him to unlock it. He'll have to prove he owns the phone though.