Maintaining a healthy weight is a difficult, but important goal for achieving overall health. Many people experience a problem similar to what you describe--an inability to lose weight on the abdomen or waist. I would encourage you to discuss this with your primary doctor
. He or she can help you with healthy techniques to achieve your goal.
To answer your question: the most common reason is that it is more difficult to notice weight loss
on your abdomen. Legs, arms and face have distinct bone structures underneath them, and therefore with generalized weight loss these areas alter their appearance first. In contradistinction, the abdomen covers soft tissue (organs like the intestines, liver etc) and therefore there is no sharp structures to show the difference. Weight loss is mostly generalized, the body will remove its energy stores (fat) in an equal pattern. Patience is the best prescription--keep with your weight loss plan!
That being said, some women keep more weight on the abdomen. This is known as the "apple" body type, which is in distinction to the "pear" shape where there is weight on the hips/thighs. This is thought to be because of a relatively higher level of androgens (hormones). It is not thought to be a disease, but just variation.
A rare disease that can cause weight gain in the abdomen but thinning elsewhere is Cushing's disease. This is a rare excess of steroids (either from medicines or from an abnormal secretion by the body).
Talk with your doctor. Most likely, you need to continue with your current strategy. Good luck!