Hey @thuy
When you exclude a segment from a campaign send, Klaviyo only removes the overlap between your recipient list and your exclusion segment. So if you're sending to a list of, say, 2,000 people, and only 100 of those 2,000 also appear in your 6,000-person unengaged segment, Klaviyo excludes exactly those 100. The other 5,900 unengaged profiles aren't being missed. They were never on your sending list to begin with.
if you can share a screenshot of the definition you used to build your 90 day segment, that would help me further inverstigate this. maybe you are using OR and AND condition incorrectly. ideally your segment should look like
received emails atleast 5 in the last 90 days
AND
placed order zero times in the last 90 days,
AND
Clicked email zero times in the last 90 days,
AND
Opened email zero times in the last 90 days,
AND
Active on site zero times in the last 90 days,
One more thing worth checking: make sure your unengaged segment uses the same 90-day window and event criteria as your engaged one, just inverted. Mismatched time windows between the two segments can also create the kind of gap you're seeing.
let me know if you have any follow up questions. I hope this helps and thank you for sharing your question here in the community. If you are still stuck, feel free to schedule a call with us - https://flowium.com/lp/klaviyo-agency/
Cheers,
Arpit