Like what silkstone said, the reason the naked flash is creating less of a shadow (or none), is because there is absolutely no (or a negligible amount) of light going directly to the subject, but with the diffuser, some will come out of the sides (diffuse....
) and go straight forward. If I were you, try doing that exact setup, and turn the flash to face slightly up and a bit more slightly to the left, and if that fails, switch to manual
