You may not be receiving messages from Facebook Messenger due to interference from other apps.
To resolve this, watch the video and follow the steps below.
On your Facebook page, go to Settings > Advanced Messaging.
Select "Responses are all provided by people."
Below, in "Apps configuration," click "Configure" and choose Sirena's handover protocol as the main Receiver.
Close the window, and you're done! The problem should be resolved. if not. contact Support.