Why doesn't Facebook notify me?

Notifications are on and everything, it's as if there's a glitch or something. Yesterday notifications worked but I got a buzz and notification with EVERY message my friend sent me from an ok to a yes which was pretty darn annoying feeling the buzz and hearing the ring, and now the blasted thing doesn't even want to notify me! Yesterday I only turned off the buzzing from messenger and clicked the mute button on my iPhone 5s and today I turned it on again but i'm not getting the buzz or ring, all i'm seeing is the preview of a message with no light up, nothing. This has never happened with any other iPhone i've used from the 3G to the 6 Plus I gave to my sister and it's annoying me to hell.

The App is probably not picking up that you turned it back on perhaps try Removing it and Re Install it and see if that gets it to Pick up the Sound Notification again