I have a few apps on my phone that aren't showing any notifications unless I open them is their away without reinstalling the app for notifications to show
What device do you have? Does this persist after a reboot? If you let us know which device it is, we might be able to suggest a cache partition wipe, which can sometimes help various issues like this one.
The system cache is where various temporary system data is stored. Sometimes, that data can cause certain glitches. There isn't necessarily any predictable behavior to look for--that's why a cache partition wipe is always worth a try as a general troubleshooting step.
Look at your status bar--is there an icon that looks like a circle with a line through it, or a star? If so, that means that you're set to No Interruptions or Priority Interruptions. On your homescreen, press either Volume Up or Down, then look under the volume slider bar that appears at the top of the screen. Is "None" or "Priority" highlighted?
Ok, this is the last thing I can think of--boot into Safe Mode, which disables all 3rd party apps. Maybe there's an app you installed that's causing this problem. If the problem goes away in Safe Mode, then you know it's a 3rd party app--the challenge is figuring out which one. https://support.google.com/nexus/answer/2852139?hl=en