Kelley, on 1200 calories a day you would be losing more weight than on 1675, but you would probably be losing some muscle.
If you eat too few calories, you do lose weight, just not as much as you'd expect. Basically, there isn't much point in reducing calories below 1000. But still, your body has to run on something, and the less calories you eat, the more it will take from your body. If your calorie intake is severely restricted, your body will take energy from anywhere in your body, including fat, muscles, and organs.
Anyway, if you're only eating 1200 calories a day and not losing weight, I would get a thyroid test. Get your doctor to order blood tests for all the thyroid hormones (not just TSH). Also, get a blood glucose test. If your blood glucose levels are high, you might find it hard to lose weight, and it can lead to type 2 diabetes.