Why isn't my iPhone lighting up when I get a notification?

If you have any Apple products, you may know that when you get a notification your device lights up to show the notification on the lock screen. Well my iPhone isn't doing that. I've even got everything set to 'View in lock screen' and the Notification Center is set to 'on'. This has been happening for about 3 months. I have restarted it, shut down for 10 minutes, and everything. But it won't seem to work. So every time I'm texting someone, I have to keep pressing the lock screen to make sure I'm getting notifications. (And when I do that, I do have notifications) So hopefully, someone can help me fix my problem.

It will only show 5 or 10 notifications then if its showing too much from facebook or an app or whatever it will stop

Go to:
1-settings
2-general
3-accessibility
4-LED flash for alerts(turn on)
Hope I helped!