Your device manufacture might have mandatory application running in the background such as battery manager/ mobile manager/ Auto-start manager which by default will stop notification of some applications you have already downloaded. Here is how you can check:
Go to settings –> advanced settings –> battery manager –> protected apps, then find the app you want to see notifications from, and protect it. This is “whitelisting” the app so device’s overeager software doesn’t shut it down for no reason. Sometimes the applications are shown in the application list as mobile manager.
Go to settings –> apps –> advanced –> ignore battery optimizations, then find the app and ignore it. Don’t be tricked by the misleading wording, “ignoring” the app actually means to let it run, because you’re telling the battery optimization function, aka Doze, to “ignore” that app.
Go to settings –> notification panel & status bar –> notification center, then find the app, then activate “allow notifications” and also “priority display”. You have to activate the priority part too to make sure you get notifications.