Many causes. You may be eating too much for the amount of exercise you are doing or are not exercising with enough intensity or enough time to get the benefits. Could also be hormonal, thyroid, medications. See your doctor and get checked out if you have not recently.
You eat too much. You must walk/jog/run 36 miles to burn one pound of fat. It is almost impossible to loose weight by exercise alone. Eat less than you burn, the weight comes off. Slowly.