Whats wrong with me?

I had an underactive thyroid 9 years ago and put on about stone it weight. I have been trying unsuccessfully in the last few years to lose this weight. My thyroid is now under control. For the last month I have been on a no carbs diet (Atkins?) but I have only lost about 2 pounds. I also do 100 sit ups per day. I eat 4 bowls of rice crispies per day with soya milk and a main meal of chicken or pork and lots of vegatables. Why isn't this weight coming off? Does anyone have any advice? Thank You