I've been a CC member since December 29, 2008. I'm a 5'11" female & currently weigh 197 lbs. I usually have a calorie deficit of between 400 and 700 calories per day plus at least 1/2 hour of exercise around 3 times a week. My weight log looks like a seizmograph! I'm constantly up and down with no real, lasting losses. I'm far from perfect, but even with an indulgent day here and there I should be losing something. I've had my thyroid levels checked several different times & they're always ok. What the heck is going on???
I have similar stats as you, 5'11 female, 212 lbs. I have already lost a lot of weight, but am aiming to reach 175 lbs. I have a naturally muscular body (although it was mostly hidden under all that fat, haha) but a slow metabolism, so I have a difficult time losing weight.
I lost the majority of my weight by eating 1000-1200 calories a day and working out (cardio once a week for 60 minutes), lifting weights (moderately, 30 minutes maybe twice a week) and walking every day. Actually, I used to tell people I was on the "shop-til-you-drop" diet because I would go to stores and walk walk walk... until I dropped the weight! I would browse occasionally, but the main purpose of being there was to walk. In fact, if I had items to purchase at the store, I would purposely walk from one side of the store to the other only getting one item at a time.
Are you sedentary at your job? Try adding a LOT of walking: every chance you get. Park far away from the entrance to your job or stores, always take the stairs when it's safe to do so, and make a point to walk anywhere you can at every chance you get. Perhaps you should try a pedometer?
Hope this helps, or at least inspires you!
By the way, I just joined yesterday... anyone reading this, feel free to keep in touch!