It'll probably take a while for your body to get used to a different way of eating. A STS is fine for your first week, not everyone loses loads the first week - I certainly didn't and I'm now at target.
Keep up the good healthy eating and make sure you have your syns (at least 5 a day).
It's better to look at your losses over a month rather than weekly as your weight varies by several pounds over the day never mind the week!
Denise