Review how the group expands
Confirm whether the group is static, dynamically filtered, nested, or synchronized from another directory. A member may no longer match a filter, a nested group may be excluded, or a synchronization delay may leave the local list out of date.
Check sending permissions and recipient restrictions. Some systems prevent external delivery, hide members from the sender, or moderate messages before release; these controls can change who receives the message without changing the displayed group label.
Trace delivery for the missing address
Verify the exact address and look for a rejection, suppression, bounce, quarantine, or moderation event in the available logs. A message marked sent can still have been refused later by a receiving server.
Use a small controlled test after correcting membership or policy, and ask a recipient to confirm delivery. Avoid repeatedly resending to the full group until you know which addresses received the original, since duplicate messages can confuse recipients.