This is a first. I will run through my set up and then the "error".
At 5:30pm, Sunday through Saturday an scenario in Make.com will trigger to gather the information of appointment data (Acuity Scheduling) of the next day and send an appointment reminder to either confirm, re-schedule or cancel their appointment. Every patient in Acuity must have their own phone number in order to send the reminder through WhatsApp. I first look for the contact in Manychat with the phone number as a custom field. If the contact doesn't exists, it is created. After that, I update some of the contact's custom fields that contain the appointment data and finally I send the message using the "Send Flow" module of Manychat in Make.com.
This has been working good for over a month now. Yesterday the workflow ran as usual with no errors.

I get notified that some patients didn't receive the reminder and here's the kicker: in the chat with the contact the data is updated but the message wasn't sent.

I've tried looking for any error messages in the log or email and nothing appears.
And I also got everything in check:
- I have enough funds on the Manychat Wallet
- Phone number is correct and complete
- WhatsApp subscriber was created correctly as any other previously
- I'm using a paid message template for every time a message is sent.
I can't find or think of a possible root to his problem. Any help is appreciated