You're talking about the "Insightful Listener, Observant Eavesdropper" achievement, right? I believe you have to hit all the conversations in the same playthrough for it to count.

Check this guide. At the end is a list of every person you need to talk to.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=784990868
If you've already done this , then do it again. The last achievement didn't pop for me until my third time through.