Facebook Messenger do not send notifications in iOS 9, even more than that, it doesn't even display the number of unread messages in the badge of the iconic application installed in Apple's iDevices.
I noticed the problem after I realized that I have several unread messages and no notification from Facebook Messenger, while the icon in Springboard tells me that I have only one unread message, although in reality there are many more.
Although the application has message notifications enabled both in the entire iOS menu and in the LockScreen, unfortunately it refuses to send alert banners for notifications, at least until a reinstallation made through the App Store.
In my case, this little fix worked and you can try it to fix the problem, if you encounter it, because it is not absolutely necessary to have a problem with Facebook Messenger, although I have encountered it with many people.
Although you would be inclined to say that the problem is related to the Facebook Messenger application, in reality iOS 9 has a bigger problem with notifications, but I will talk about that in a separate article.
