Your main subject is too small, your composition is flat.....
You must change totally your technique: put your main subject nearer of your lens. The good distance must take into account the size of your subject. Also don't forget to put the axis of light flash on the subject at the fisrt position. The background is lighting by the sun.
example in a swimming pool of a main subject.
a composition with flash and "sun light": you must have a difference of one aperture.
