wrong collision when loading and instanciate a prefab

i have a prefab with a mesh collider. if i create a new scene with the prefab inside the collision with my character works fine. If a create a new empty scene and the use resource.load and instanciate to put the prefab in the scene the collision doesn't work. This happens only on the ios device, the editor works fine. The prefab loaded and the manually placed have a different mesh(?) i don't know why. the prefab is the same, the mesh looks the same but has different name : the loaded one is 'road', the manually placed 'combined mesh'. Any helps? how can i fix the problem?