Hello przpect,
Ensuring you're getting notifications from Facebook Messenger and other applications is important. Although Notifications may be enabled in Settings > Notifications, the settings can be customized for each individual app. You can find those steps listed below, and in the following article. The following article also provides additional information about customizing and using notifications on your iPhone to resolve this issue.
"Change how notifications appear on the Lock Screen"
- "Go to Settings > Notifications and select an app.
- Tap Show Previews and choose an option."
Use notifications on your iPhone or iPad
Take care!