It is a frequency guard: each channel has its own Smart Sending window (by default 16 hours for email and 24 hours for SMS and push), and a contact messaged within that window is skipped by the next send. It protects against fatigue and unsubscribes, but it can also silently suppress messages you intended to send, and skipped messages are not rescheduled.

In practice

Know where Smart Sending is on and what each channel’s window is set to. It quietly reduces sends, which is usually good for engagement but can undercut a time-critical message if you have not accounted for it. It affects whether a message sends, not whether someone enters a flow, so check it before debugging a message that looks missing.