<s>Are you trying to use a Spiral Stair builder brush to subtract a stairway out? That way always tends to result in heartbreak, as the faces on the inside don't seem to line up exactly and you're left with a micro-thin (but completely-blocking) vertical surface at every step.</s>
(wait, I apparently can't read the word "add" in your post... I am the stupid)
Maybe you've set the step height too high? If the step height is greater than the step length, you can start to run into the problem of unclimbable stairs. If you're using a Curved Stair brush, reducing the number of steps will do the trick.