help me with 3ds max plz

[DJS]

i took the sentinel from gta sa and deleted the damaged and ok front and back bumpers. then i took  a damaged and ok front and back bumper thats used to mod street racers, i then renamed the new bumpers exactly the same as the ones i deleted and even made them he same colour. but when i try to export the model with the following settings, no new dff is created . plz tell me what im doing wrong