Mesh Collider collision problem

I am generating a mesh surface for my player to walk on in chunks; for each chunk, the code below is run. My issue is that the player (also tested cube & sphere with and without rigidbody) goes right through the floor (also raycasts won't collide with the mesh). I have scoured the internet for a solution; OnTrigger is not enabled and the mesh is in the default layer. However, the part I really don't understand is if I pause the game and manually add a mesh collider to the floor, the objects will collide with the floor then. I've compared the two colliders (script generated and manually generated) and they appear identical. Any pointers would be greatly appreciated.

 meshFilter = GetComponent<MeshFilter> ();
 meshRenderer = GetComponent<MeshRenderer> ();
 meshCollider = GetComponent<MeshCollider> ();
 
 // if there's not a mesh filter, add one
 if (meshFilter == null) {
     meshFilter = gameObject.AddComponent<MeshFilter> ();
 }
 
 // if there's not a mesh renderer, add one
 if (meshRenderer == null) {
     meshRenderer = gameObject.AddComponent<MeshRenderer> ();
 }
 
 // if there's not a mesh collider and we want one, add one
 if (meshCollider == null && colliderOn) {
     meshCollider = gameObject.AddComponent<MeshCollider> ();
 }
 
 // If there's a mesh collider and we don't want one, destroy it
 if (meshCollider != null && !colliderOn) {
     DestroyImmediate (meshCollider);
 }
 
 // Get the mesh
 mesh = GetComponent<Mesh>();
 // If there's no mesh, add a mesh
 if (mesh == null) {
     mesh = new Mesh ();
     meshFilter.sharedMesh = mesh;
 }
 
 if (colliderOn) {
     if (meshCollider.sharedMesh == null) {
         meshCollider.sharedMesh = mesh;
         meshCollider.material = null;
     }            
 }
 
 meshRenderer.material = chunkMaterial;