Why doesn’t my lock screen on iPhone turn on?

Usually when i receive a notification on my phone, the lock screen lights up and i can see what notification i got but for some reason, i have been noticing that i hear the buzzing when getting notifications BUT MY SCREEN IS OFF IT DOESNT TURN ON AT ALL. ALL MY APPS HAVE THEIR NOTIFICATIONS TURNED ON BUT IM REALLY WORRIED AS TO WHY MY SCREEN stays black but i get notifications ..