Variety. Weight loss is a tough thing to conquer. Doing situps alone will not make your abs flatten out. You need to lose weight. Jogging and skipping rope are great exercises that can help to reduce weight. Try a variety of ab exercises, don't just stick to one.
A tummy problem? A woman who is a size 10, appears to have large abdominal muscles, but still seems to have a big tummy, should be evaluated by a primary care doctor. She may be normal and just built a certain way. However, she could also have a problem inside the abdomen such as distended intestines, fluid in the abdomen, an enlarged liver, a tumor, etc..., that is making her tummy look larger than it should be.