When do segments update?

Segments maintain themselves. The Segments page describes them as fully automatic — nobody adds or removes people, and there is no refresh button to press.

Last evaluated

Each segment shows Members and Last evaluated. Together they answer the question people actually have: is this number current?

Members is the count as of that evaluation, not a live figure recalculated as you look at the page.

So there is a lag

A customer who places an order right now does not appear in a "recent buyers" segment instantly. They appear at the next evaluation.

This matters in two places:

• Sending immediately after a change. If you have just imported contacts or changed a rule, check Last evaluated before launching a campaign at that segment. Sending against a stale evaluation is how you email 40 people instead of 400.
• Automations triggered by entering a segment. The trigger fires on evaluation, not on the underlying event, so a "became a VIP" flow starts shortly after the customer qualifies rather than at the moment of purchase. For genuinely instant reactions, a custom-event trigger is the right tool — see Automation triggers.

Zero members

Either the rule matches nobody, or nothing has fed it yet. Both look identical in the table.

Check Last evaluated first: if it has never been evaluated, wait rather than rewriting the rule. If it has been evaluated recently and still shows zero, the rule is the problem — or the data it depends on is not arriving.

Next: open Segments and compare Last evaluated against when you last changed the underlying data.