I've read that three times, and it made a different sense each time.
A "down stair" is a hole in the floor, the barrier between the level you are on and the next level down. But alone it's only half of what's needed for a path.
An "up stair" will connect the level you are on with a "down stair" directly above it. But alone it's only the other half of what's needed.
An "up/down stair" will do both - put a hole n the floor, and connect to a "down stair" above it. But it's the bottom half of one path and the top half of the other path - not a solution by itself.
If you have a down stair (which includes the "down" part of an updown) but no upstair below it, you have no path, just a hole in the floor (or a hole in the ceiling if thinking about it from one level down).
Likewise, if you have an upstair (which includes the "up" part of an updown) but no down stair above it, you have no path, just a stair leading up to a blank ceiling. (See "Winchester Mystery House").
What it sounds like is that, somehow, you swept an area with the <d, z> designation and removed some upstairs (maybe while trying to remove some slopes?), and so have no way to get up to the hole in the ceiling. If that's not the case, you've done some truly creative remodeling.
The suggestion to draft a dwarf and march them around is a good one. See where they will go, and where they won't, and how they insist on getting there.