I am getting notification sounds, but no pop-ups or history. It says the notification service in not available, and it looks like do not disturb is on but I can't turn it off. I'm using kubunutu 20.04. I think installing kdocker might have caused this, but I've since uninstalled/autoremoved kdocker and restarted with no change. Reading similar posts online, there may be a different applet trying to control notifications (which is why I uninstalled kdocker) but I can't find anything I have installed that would do that. I've included a screenshot of what it says in the system tray.
Thanks.

What are your settings in System Settings -> Personalization -> Notifications?

There are two configuration files for notification, one is ~/.config/plasmanotifyrc which holds specifications for non-KDE applications, the other one is ~/.config/plasma_workspace.notifyrc.
I suggest renaming both, log out and back in again and see if this helps.

I tried this but it didn't change anything.

Changing video drivers appeared to fix this issue. I had been using the kisak mesa PPA drivers, but uninstalled them due to video lag issues. My notifications work now too. Notifications had been working with the oibaf mesa PPA drivers too, only had issues with kisak.