sirena

I had surgery Aug.3 this year. By my first fill on Sept.9., I had lost 24lbs. I was told by my dr. that I should loose 15lbs more by Nov.11, when my next fill/adjustment is due. I am have lost only 10lbs and have been stuck with the same weight for a while. I do 2.4 to 3mls twice a week on the treadmill at the gym and try to walk other days during the week. Any idea why I am not loosing?