Count your calories. On WW, you can expect to lose around 2 pounds a week but that is if you really WORK the program. Most people start out being super-careful of points (calories) but time wears on and they slack off. That's the challenge of losing weight, right?
So 15 pounds is a little too much to lose in such a short time, realistically. Really count your calories if you're so motivated, but calories is basically what it is - carb restriction just ends up being calorie restriction in the end. See what works for you.