A picture could help, yeah.
That being said, my general observation is, that NPCs don't go up stairs, if the room otherwise continues, or how to put it, so, they have a tendency to go upstairs about 4 blocks height, then jump off the stairs to continue walking to that direction in the room.
If the stairs are outside of a building, they usually walk up and down, however, they have a tendency to completely jump off the stairs, when trying to go back home (at 7:30 p.m. or any other reason, like rain). they often jump off, even if they will end up falling down way below their house and won't be able to get back, except via teleporting.
you can, however, get NPCs to walk up stairs and go up the next floor of a building, by placing blocks (preferably hammered to a slope) under some part of the stairs (starting around 4 blocks height, so the player can walk under it). That way, they can't fall off, and end up going upstairs. but then they won't come downstairs* until it's 7:30 p.m. or anything that causes them to return to their rooms.
*if the part right above the stairs is a row of platforms, functioning as the floor there. because NPCs never go down on platforms, except when returning to their assigned room.