The NPCs i am talking about smith, jeveler and other nomad romani woman. I had lots of unfinished dialogue options with them, left them for the endgame then killed diablo and continued to act v and voila, whole dialogue options are gone. I can’t learn how their stories end now, disappointing…
This was not a case between when you pass from act ii to act iii etc.
You should be able to go back to Act 4, or any other act, by clicking on Game Settings on the main menu and then clicking the Change button on the new pane that will pop up. Be careful not to click on the Reset Quests button, if it’s available – it probably isn’t because you haven’t done the whole story yet, because that would throw you back to the very beginning of the story.
This is not a bug. There are some Conversations, for many of the NPC Achievements, that can only be accessed at certain times in the Campaign. If you miss your “window”, they’re gone.
As Boubou pointed out, you can reset your game to the proper Quest and get the Conversation at that point.