Hey, ​@revoul 

Are you using the free or pro plan?

→ If you’re using the free plan, make sure that your account is under the limitations. You can check everything here: https://manychat.com/pricing

→ If you’re using the pro plan, make sure that you have all the permissions set correctly. Please, read these help articles: https://help.manychat.com/hc/en-us/articles/14281479971996-Automations-troubleshooting and https://help.manychat.com/hc/en-us/articles/14281308423452-Instagram-automation-troubleshooting

If none of this helps, please feel free to give us more details about your automation and we try to help you further.

I had the same issue, and FINALLY figured it out… there are settings in META that all have to be set ‘just right’… 
From your business page, go to Settings > Page Setup and then this Messaging section: 
 

In Advanced Messaging, ensure the Apps are connected:

Manychat for FB Messenger

Manychat IG (IG App) for Instagram

Then in App Settings make sure the respective apps are set as the
Receiver” for Messenger and IG

THEN go back to the Messaging section, and open the Conversation Rounting for each:
 

And set the Default Routing App to ManyChat

TIP: You may also need to go to your FB Personal Profile and go to Settings & Privacy > Business Integrations > and edit the ManyChat settings to ensure ManyChat has permissions to monitor and reply to messages, etc. 

That's correct ​@Conrad!
As I mentioned to ​@revoul the step-by-step guide is in this link: https://help.manychat.com/hc/en-us/articles/14281308423452-Instagram-automation-troubleshooting

​@revoul Have you tried to follow these steps?