A few things to check:
Make sure the stairs have colliders that connect to the floors (sometimes there are gaps that can cause problems).
If the stair colliders are just a slanted box (instead of individual steps), make sure the navigation mesh walkable slope is high enough to get them (it probably is unless it is a steep stair case).
If the steps are individual colliders, make sure the step up height on the navigation mesh is large enough to take the stairs into account.
Make sure the walkable height is low enough so that it doesn’t clip the ceiling when going up stairs (sometimes it can be close).
Lastly, if those things don’t seem to work, try using a simpler collider on the stairs, maybe just a slanted box. You can put it in its own layer and turn off collisions for that layer, and just make sure the navigation mesh includes it (should include everything by default).