Go to Windows settings > System > Notifications & actions (1) and enable the option to “get notifications from apps and other senders”.
Why do some apps not show notifications?
If restarting your phone didn't do the job, one of the most common reasons notifications don't show on Android is because of the notification settings of the app in question. It's possible that you may have mistakenly messed up with the default notification settings, and so now you're not receiving them properly.