Without looking at the actual files, the only thing that comes to mind is that a .obj file will make references to materials by way of an associated .mtl file. Usually the same base filename, but not necessarily. Usually in the same folder, but not necessarily. Check the content of the OBJ file. It will have early on, near the top of the file, a statement that points to the .mtl file. If that's a relative path to the file it may find it but if it's a full path C:whatever... then it may have worked where the creator used it but you'll need to change that entry if you don't have the files in the exact same location as they once stood on the creator's system.

-Philip