Right, to lose weight effectively you shouldn't go below your daily calorie requirement by more than 20% (10-15% should be sufficient).
If you eat too little (and as everyone seems to agree, that possibly is the case), your body will panic, think ooh it's a drought, I'm going to hang on to all the fat I can get until it passes - clever things bodies! So your metabolism slows down, becomes less efficient and you stop losing weight. Your body just learns to survive on less food.
So boost your metabolism, eat quality to fuel your exercise, (your SW diet will be based on someone who's not doing any/much exercise so you need to make up the extra to carry on losing) get a copy of Anita Bean's Sports Nutrition book if you want some good reading on the subject.
Good luck, sure you'll get there in the end. After all, the aim should be to develop a healthy, sustainable way of living rather than to lose lbs, that's just a by product.