I lost 3 stone with SW. As MrsSalvatore said it's not how much you eat, it's what you eat. It's a low fat, low sugar diet.
My friend failed to lose weight but she was also struggling with thyroid problems.
She's the only person I know who hasn't lost weight. Of those in the group people who didn't lose weight some weeks could put it down to not sticking to the plan, skipping meals, or not eating enough, (your body goes into starvation mode and stores food) or they were eating things that they thought were good, but weren't (like smoothies, fruit is free, but you burn calories by chewing and digesting it. A smoothie it takes no effort to digest so becomes syns.)
Or in my case I went back to lose the post Christmas excess, did great for 2 weeks then maintained or gained for 3 weeks. Got really annoyed... then realise I was pregnant. :)
I recommend going to a class, you can sign up for 6 weeks but you only pay for 5. 6 Weeks is enough to work out if you're going to stick at it. Also the first week you get all the information but it might not make sense so you can go back and ask questions and you'll learn from others.
I couldn't afford to pay either, but that's what made me lose the weight. I objected to paying every week so stuck to the diet. The faster I lost the weight, the sooner I could start going for free. :)