Glitch in nav mesh when adding my own assets

First time I am using UE4 and I am having a problem. We have created some roads with sidewalks where people are going to walk on the sidewalks. We created the static mesh in Blender and added collision boxes to the road component’s blueprint in UE4 to mark the difference between the road and the sidewalk. But when we add this road component to a level and encapsulate it in a NavMeshBoundsVolume we get some weird glitches. There comes gaps at random which our AI people can’t move past.

In the upper picture you can see the gap, and in the lower you see the component without seeing the navmesh (P). The gap comes at random, if I add the exact same component at another location in the same level, there is no gap, but adding another one might cause a gap to appear… The same thing happens if I only add the static mesh, so I suppose it does not have anything to do with the blueprints.

[Here is a link][3] to a small project i created with a few components which shows the “gap” i’m talking about. If you open TestLevel in Content → Levels I have a small map where you can see the glitch, both when using a road as a blueprint and when it is a static mesh.

Does anyone know why this is happening and how to fix it?