@mermaidzeli
When flow is triggered by segment (or a list) people who are already in that segment (or list) when flow is created are not triggering the flow. As you can see in the trigger name trigger is when someone JOINS list or segment not when they are in it.
So your problem here is that those 5000 people were already in segment when you started the flow so only new people who join the segment will trigger this flow.
There are 2 ways that I can think of that you can fix this..
1st is if you don’t have dynamic coupon codes or dynamic product blocks in this flow, you can backdate the flow and all the people in that segment can go through the flow.
2nd is create new flow started with new empty list (make sure to set single opt in when creating this list), and than when that flow is ready and set live (but remember it must be live first) download the people from this segment and upload to that new list. So when you add people to that new list this new flow started with that new list will trigger for all of the people added there..