ah, the old "you're not eating enough" reason..
you need 1400 calories a day ( ladies ) to lose weight..
any less and your body goes into "starvation mode" ( on a normal diet anyway, there are specific diets that trick do this but SW isn't one of them.. ), and you stop losing weight.. your body tries to hold on to everything it can.. until it can't anymore and then it starts breaking down muscle rather than fat so to keep your internal organs protected..
slightly larger portions or more syns will give you those extra calories you need to get you into the right "zone" to keep you losing weight..