Why are some tummy tucks box shaped/no waist line?

there are 3 possibilties :1 this maybe the natural shape of the patient 2 insufficient tightening of the muscle sheath (plication)3 patient is still overweight with significant internal fat around the gut etc.

Thanks for your inquiry.  Each person's undderlying shape is different with some individuals having narrow hip bones or wider rib cages. If you are concerned about your potential result being boxy, please be examined by a plastic surgeon to  possibly advise you.  

A lot of the boxy of the abdomen, hips, and waist depends on your basic anatomy.  If you have a large rib cage and narrow hips, it can be harder to achieve a curvy appearance.  That said, liposuction of the hips, flanks and waist plus thoughtful sculpting of the abdominal muscles can go a long way to getting you the appearance you desire.