Why sometimes I can't clear some tier 6 quests?

[nico_zip]

I go into the quest, infestation tier 6 on grove high, kill every zombie it spawns, walk every corner of the building reach loot, quests isnt clear, take a whole 3 more tours into the school, mark every room, etc.

Yet no zombie is left standing but the quests is uncompleted. I would believe is my fault but first I have cleared this quest before and second it has happened to me in multiple quests randomly, i'd say i could finish like half of the tier 6 quests i take.

[Khissi]

Some zombies only spawn when a certain trigger is activated, whether it be that you've entered a certain space, crossed a trigger path, fallen through a ceiling, etc and will NOT appear unless the trigger was activated properly, as in, you've followed the pathway. Sometimes, the path is not as easy to make out, and you may have strayed, and this may prevent the trigger from activating.

I'm not saying it's the best way to have done this, In fact, I abhor it because I love stealth builds, but I am saying that this is how it works. The more times you do a particular POI, the better you begin to understand its triggers and path, and you'll find ways to prevent that from happening.

[Trooper Bri]

You've probably got a zombo stashed in a wall or ceiling that isn't triggering.

Did a T5 at the Haven Hotel recently and had the yellow dot nearish to the loot room.
Thought it was going to be a case of getting the yellow dot bug, but no. I eventually found a wall plug, and there was my last guy. He just refused to wake up for whatever reason.

Thank god. I'd have hated to clear that whole place and get robbed out of my trader reward of 5 duct tape...

[Macdallan]

Broken trigger points are a likely culprit as the other responses have indicated.

I had success re-setting a trigger point for what turned out to be one single enemy that I couldn't find. I did this by walking out of the POI (but staying within the POI radius) and then going all the way back through to the "end" area.