TL;DR: The next time you build a flow and find that your existing segment members aren't entering, the short version is: that's expected, and Add Past Profiles is your first stop. The slightly longer version is that flows and campaigns are doing different jobs. Flows respond to change; campaigns reach whoever's there right now. Once that model clicks, it's usually clear which tool to reach for.
If you've set up a segment-triggered flow and noticed that the people already in your segment aren't receiving it, that's not a bug. It's how Klaviyo flows are designed to work. Flows fire on entry into a segment, meaning a profile has to newly qualify by crossing into it. People who were already there when you built the flow didn't cross anything; they were already in. This article covers the three situations that creates, and exactly what to do about each one.
Why flows don't send to people who are already in your segment
A segment-triggered flow fires when something in a profile's data changes and causes them to meet your segment conditions for the first time, or again after they've left. That moment of crossing in is what Klaviyo uses as the trigger.
Think of it this way: the flow is listening for the door to open, not monitoring everyone already inside the room. If a profile was already in your segment when you activated the flow, Klaviyo has no trigger event for them. Nothing changed. They were already there.
What counts as "newly qualifying" in practice: a profile property updates, a purchase is placed, an event fires. Something changes in their data that causes them to meet the segment conditions. That data change is the trigger. Profiles who met those conditions before the flow existed simply don't have one.
This is worth understanding clearly, because it affects three distinct situations operators run into regularly.
Three situations this creates and what to do about each
Situation 1: You just built a flow and your existing segment members aren't entering it.
This is the most common version of the confusion. You create a flow, point it at a segment that already has thousands of people in it, turn it on, and nothing happens for most of them. Only new people entering the segment start receiving messages.
The native fix for this is Add Past Profiles. In the flow builder, you'll find the Add past profiles icon button in the top right corner of the Trigger card. This is Klaviyo's built-in option for getting current segment members into a flow that was built after they already qualified.
When you use Add Past Profiles, you'll be asked to choose between two placement options:
- Throughout the flow — Klaviyo places each profile at the point in the flow that corresponds to when they originally joined the segment. Someone who joined your segment six months ago might land partway through; someone who joined last week would be closer to the start. Use this when the flow's content is tied to lifecycle stage or time-since-qualification.
- At the beginning — Every profile is treated as if they're entering the flow right now, starting from step one. Use this when you want everyone to receive the full sequence regardless of when they qualified.
Whichever option you choose, profiles added this way still move through the flow according to its configured timing. Time delays apply from the point they're placed, so if there's a 3-day delay before email two, they'll wait 3 days.
One important distinction: Add Past Profiles is a separate action from organic flow entry. Re-entry criteria, the setting that controls whether profiles can enter the flow again if they leave and re-qualify, does not apply to profiles added this way. It only governs organic entry.
Situation 2: Someone left the segment and came back. Will they re-enter the flow?
Maybe. This depends on how your Re-entry criteria is configured in the flow trigger settings.
Re-entry criteria is available to all Klaviyo customers who use Flows. To find it, click on the trigger in your flow builder to open the trigger settings panel and you'll see the Re-entry criteria option there. You can read more about how to configure it in Klaviyo's guide to segment- and list-triggered flows.
By default, most segment-triggered flows are set to no re-entry, meaning a profile who leaves and re-qualifies won't enter the flow again. If your use case requires re-triggering, say a win-back flow that should run again if a customer lapses a second time, you can adjust this setting to allow re-entry.
One thing that does not trigger re-entry: a profile simply remaining in the segment. Re-entry only happens when segment membership actually changes. Someone drops out and then qualifies again. Staying put doesn't count.
Situation 3: You want to send a one-time message to everyone currently in the segment, outside the flow.
This is a different goal entirely, and the right tool here is a campaign, not a flow adjustment.
If you want to reach your entire current segment with a specific message, a product launch announcement, a promotion, a one-time re-engagement, send a campaign targeted to that segment. The campaign goes to everyone who qualifies at send time, regardless of when they entered the segment or whether they've been through the flow.
This is especially useful when the message doesn't belong in the flow sequence. It's not part of the automated journey you've built; it's a standalone send. Using Add Past Profiles to push people into a flow just to receive a one-off message is messier than it needs to be. A campaign is cleaner.
How to use Add Past Profiles and what it actually does
Since Add Past Profiles is the primary fix for Situation 1, it's worth being clear on what it does and doesn't do.
What it does: It inserts current qualifying segment members into an existing flow, using one of two placement options (throughout the flow or at the beginning, as described above). It's a deliberate, one-time action, not an ongoing setting.
What it doesn't do: It doesn't make those profiles organic entrants. They won't be subject to Re-entry criteria because they didn't enter through the trigger. If you later change your segment definition or re-entry settings, those changes won't affect profiles who were added via this path.
When to use it: Any time you've built a new flow and want to include people who were already in the segment before it went live. It's also useful when you've made a significant update to a flow and want to run existing segment members through the new version.
To access it: in the flow builder, look for the Add past profiles icon button in the top right corner of the Trigger card. For full steps, see how to create a segment- or list-triggered flow.
When to use a campaign instead
A campaign is the right call when:
- You want to send a single message to everyone currently in the segment, with no ongoing flow involvement
- The message doesn't fit the flow sequence. It's timely, one-off, or promotional in a way that doesn't belong in an automated journey
- You want precise control over send time for a specific moment (a flash sale, a product drop, a deadline reminder)
Campaigns send to whoever qualifies at send time. Flows send to whoever newly qualifies from that point forward, plus whoever you explicitly add via Add Past Profiles. They're complementary tools, not competing ones.
A quick way to check who's actually entering your flow
If you're not sure whether the issue is "no one is entering" vs. "people are entering but getting filtered out before send," the trigger preview tool is the fastest diagnostic.
In the flow builder, click on the flow trigger to open the settings panel, then click Preview. For list- and segment-triggered flows, it shows the last 10 profiles added to the segment and whether each one passed the profile filters, along with the reason if they didn't.
This is particularly useful when you have profile filters configured on your flow. It's easy to set up a filter that makes sense in theory but quietly blocks more people than you intended. The preview surfaces that immediately, without you having to wait for live sending data.
Note that this tool works for list-, segment-, and metric-triggered flows. It's not available for date property-triggered or price drop flows. For more on troubleshooting list- and segment-triggered flows, Klaviyo's help center has a full breakdown of common issues.