Why Do Some Women Have Really Flat Stomachs After Having Had a Tummy Tuck and Some Women Are Not As Fortunate?

Thanks for your question.
When an abdominoplasty is performed in most cases the abdominal wall muscles are tightened to flatten the abdominal wall.
  When I tighten a slender person they tend to stay quite flat because the weight inside the abdomen is not as heavy.  A medum weight person will tend to stretch more over time and find a new equilibrium that is still an improvement. In a very heavy person the weight inside the abdomen  is greater and the overweight person may regain most of the roundnes over time. 
The body is smarter than the doctor and over time relaxes to where it is comfortable.  That is why it is always better to be healthy and if possible slender as you embark upon tummy tuck.
Best wishes.
 
 

Factors that can affect whether you bulge or not after a tummy tuck (abdominoplasty) include: 

1. Whether you were flat to start with (most important)

2. The extent of muscle tightening by your surgeon - Experience and Technique dependent

3. Whether you are slim or obese (BMI) - heavier patients have increase weight of abdominal organs and intra-abdominal fat pushing out

4. Whether you had liposuction done at the same time and how much fat was left behind

5.. The resting tone of the abdominal muscles - Doing core exercises like crunches helps a lot

6.  Weight Gain after the procedure

7. Familial tendency (check out the shape of your parents or older sibling )

Some patients deposit fat inside the abdominal cavity (andromorphic fat distribution).  This is seen much more commonly in men and is colloquially referred to as a " beer gut".  This fat cannot be reduced by liposuction (it's too deep).  A tummy tuck can only compress it so much so the results seldom leave the patient with a flat stomach even after a very thorough operation.