Hi @kmaleki thanks for posting in the Klaviyo community! 

When you back populated your flow, which option did you choose? There are 2 ways to back populate - the first way is to back populate relative to when you hit the back populate button (so all people will start at the beginning of the flow) or the second way is to back populate relative to where someone is in the timeline. So if people had entered the flow 2 days ago, and there is an email on the second day, when you back populate, all subs will start at whatever email they would be receiving if they had been going through the flow. 

Here’s a bit more information on back populating from Klaviyo and how back populating works.

Something else to note: if there is a time delay at the beginning of your flow, when you back populate, people will have to wait out the time delay. (For example, if there is a 2 day time delay at the beginning of the flow, when you back populate, subs will have to wait the 2 day delay before receiving the first email. 

If none of these are the problem, would you mind posting a screenshot of your flow so we can help you determine where the issue is? 

I just noticed that the contacts I thought weren’t populating are actually in the time delay period for the second email. I think I accidentally back-populated relative to the timeline--would people who were on the list for ten minutes skip the first email? Maybe I should throw in a half-hour time delay at the very start just in case I make the same mistake again. 

Is there any way for me to get those subscribers back to the first email? I have a different path set for people who don’t open the first email, so all of them will end up going down that path if they never even received the first message.

I checked the history and actually, I did hit the ‘relative to button click’ option, so I still don’t know why they didn’t get that first message. I deleted all of those contacts and re-uploaded them, this time with that start delay. 

Update: That worked! I’m starting all my flows with a delay from now on. Phew!

@kmaleki glad you got it sorted!