NavMeshSurface ignores some walls

Hi There,

In my project I generate a map, that has floors and walls. After this is done and all are collected under a parent object, I generate a navmesh via navmeshsurface. It does work in 90% of the cases, but some walls seem to be ignored.

You can find a screenshot attached

The problem seems to come when 2 rooms have a common wall

Any ideas what could cause this?

Thank you