Best answer
Because Meta allows free-form replies for only 24 hours after the contact's last inbound message, and your automation is hitting the edge of that window. This is a platform rule, not a setting in your automation, and it applies to Instagram, Messenger and WhatsApp.
The window restarts every time the person messages you again. It does not restart when you send, and it does not restart because they opened the chat or reacted to something.
What this means for flow design:
- A wait step measured in days will usually land outside the window. The step
runs, the send is refused, and the run ends with a failure rather than a delivered message.
- Anything time sensitive belongs in the first exchange, while the window is
certainly open.
- If you genuinely need to reach someone later, collect an email address or
phone number during the conversation and follow up on a channel that has no window.
On WhatsApp there is a supported way out: an approved message template. On Instagram and Messenger the practical answer is to get a reply, which restarts the window, or use a different channel.
ChatinFlow shows the window state in the Inbox and in automations, so you can see whether a thread is still open before you build a follow-up that depends on it.