Why mesh colliders doesn't work with combined meshes using Mesh.CombineMeshes?

I have a bunch of meshes combined into a nice mesh using Mesh.CombineMeshes, to that mesh I add a mesh collider and it doesn't collide at all!

Try so set the Convex flag. Some mesh Colliders wont work depending on how the mesh is defined.

Had the same problem yesterday, had two meshes using for colliders, separe work perfectly combined wouldn't work. My colliders once combined also didnt show the greenish collider outline in scene view after combining any more.

My fix was using Mesh.Optimize, after combining:

    CombineInstance cmb = new CombineInstance();
    cmb.mesh = colisionMesh;
    cmb = new CombineInstance();
    cmb.mesh = walkMesh;

    Mesh col = new Mesh();
    col.name = ModelName + "_C+W";
    col.CombineMeshes(combineInstances.ToArray(), true, false);
    //foreach (CombineInstance c in combineInstances)
    //    UnityEngine.Object.DestroyImmediate(c.mesh);

    MeshCollider mc = (MeshCollider)g.AddComponent(typeof(MeshCollider));
    mc.sharedMesh = col;
    return g;

ALTHOUGH i think it welds some vertices etc... so it kinda ruined my perfectly setup collision meshes :( anyone know is this a known bug in Unity, or? Note: combining meshes and displaying them instead of using as colliders works perfectly...

This is a known bug in Unity 2.6, Mesh.CombineMeshes will not generate the collision information for the combined mesh. I believe this is fixed in 3.0, though.