Navigation is not applied on stairs

I have created a stair using 3dsmax and i imported it in my project, applied vertex collision and Complex to simplr colllision and converted it to a blueprint.
When i apply the stairs (mesh and bp version) inside my project the nav mesh green color is not applied properly and some gaps divide the green color section resulting to immobilization of my ai characters (the ones under the stairs) due to the lack of a proper navmesh path.
What are the options of the navmesh or the recast navmesh that need to be adjusted in order to cover the entire stair with the navmesh? i have made many adjustment with no success.

Thank you in advance!