I built a stairway all the way down to the bottom of the map. At the very bottom I created a series of rooms I intended to use as my dungeon. Unfortunately dwarves don't seem to want to leave the dungeon once they enter. Even if they are thirsty or starving, they won't leave. First the miner had this problem, but I saved him by having him build a stairway back up and slowly he reached the surface. However, the liazon entered the dungeon and refused to leave. He stood there, staring at the walls for days until he snapped and went stark raving mad.So it looks like the dwarves are having pathfinding issues in the Z direction.
How big are your stairs? I'm ambitious about expansion so I make them as big as my hallways-- in this case the "Grand Stair" I had that went down maybe six levels was 4x4 and i didn't have any problems with pathfinding.Mind, Dwarves will look for stone, see one six floors down from where they are standing, and pick that over the one 12 tiles over from them on the floor they're on, so I'm not saying there are no pathfinding problems. I just haven't had problems with dwarves getting stuck on lower levels.