Contacts are added to a segment by a background update that runs regularly, not at the moment you save the segment. After creating a segment, wait up to two hours before checking the contact card.
The contact card lists the contact's segments under Segments. If the contact isn't in any segment yet, the Segments heading isn't shown at all.
If the segment still isn't shown after two hours, check that:
- The segment is active. Inactive segments are not updated. See What happens when you make a segment inactive?
- The segment has at least one filter. Without filters, no contacts are added automatically.
- The contact matches the segment's filters.