Quests marks don't appear

Hello there, i've been encountering this issue where i can't see any quests marks, like ? or ! on NPC's heads, but they actually have some quests assigned.

First time i've seen this in Duskwood and Redridge Mountains, and it's frustrating, because there are a lot of NPC's and i can't waste time clicking on each one of 'em for no reason... For example, in this shack in Duskwood, https://imgur.com/a/S62Tp there is a guy who gives me a quest, but there is no mark that it says to me it does that. And there are a lot of them in Duskwood, and i assume in other maps too, and i need to know how to fix it, or if it's a bug, or i can solve this with an addon.

I want to tell you that this doesn't happen in all the maps. I'm playing on TBC Outland. Thank you.