Followups are not being created because the accounts exceeeds the daily quota of logins.
Solution
Create followup messages with no daily-quota validation.
Process only messages allowed by the daily quota.
Add a banner in the conversation, in responses screen, if the account is exceeding the daily/hourly quota of logins/messages
Show the number of messages delivered by an account, day by day, hour by hour, in order to resolve the complaints "a message has been in queue for long time"