This should be posted in hints/spoilers and cheats. Btw, you don't need to put out markers by yourself, just activate the quest in pip-boy and the marker tells you where to go, what building that is. Enter building. There may no questmaker as soon as you enter the building, just search the area. Some enemies spawns and you need to be there to make it happens. It seems like you have not been *everywhere*.
Cheers,
-klevs
Sometimes the building they send you to isn't the building marked, I've had this a few times with the marker sending me to a different building.......mission completed after killing one hostile. That's another thing but it seems like you don't actually have to clear the buildings just kill a certain target and on one occasion there was a back door right to that target.
Two possible reasons for this issue to pop up:
1) You waited too long to clear out the ghouls. There's a finite amount of time you can waste before the quest auto-fails, so it's possible you got to the SDM too late (side note: I actually failed a kidnapping because of this, despite being within seconds of saving the settler. Cracked me up to be "this close" yet fail the quest).
2) You have another settlement quest open, usually a Miscellaneous "Defend a settlement" quest, which came in after the OZ quest, and thus, makes it the "active" settlement quest to Preston.
The worst part about #2 is that you can complete one quest, return to Preston, but he won't acknowledge the right quest! Annoying, because now you actually have to wait until both quests fail before he "resets".
At any rate, don't sweat these quests because they're not critical if you fail them. Sometimes, letting the game figure it out is often the best and easiest solution.