It is possible that you are not receiving messages from Facebook Messenger because there are other apps interfering with it. To modify that, check the following video and the steps mentioned.
1 - On your Facebook page, go to Settings > Advanced Messaging, then select "Responses are all provided by people."
2 - In "Apps configuration" click "Configure" and choose Zenvia Conversion's "handover protocol as the main receiver".
3 - Close the window, and you're done! The problem should be resolved.
However, if after performing these steps the problem still persists, try the following:
1 - Access Zenvia Conversion and disconnect the FB Messenger integration.
2 - Then from the FB Administrator user, access https://www.facebook.com/settings/?tab=business_tools
3 - If you only have a Facebook Messenger integration and it is failing, remove the Zenvia APP that will appear.
4 - If you have more Facebook Messenger integrations, but only one is failing, do not delete the Zenvia App, just access "view and edit", and uncheck all the options in the box that will open, the page that is not working with Facebook Messenger in Zenvia Conversion.
5 - After performing this procedure, access Zenvia Conversion and integrate Facebook Messenger again. If you have doubts about how to do it, you can click here to see the step by step of how to integrate again. Done, it should work now!
Note: Keep in mind that deleting the app will also delete the bot flow, if you have it configured.