Hi there,
I don't really know, but I saw no one replied yet, so wanted to write something.
Do you weigh enough that you have enough weight to loose? If not, that'd be one reason

1200 is pretty low for most people, although I've read that for people who are very short, it might not be low. Are you very tall?
Do you think you could be gaining muscle instead of loosing weight?
Do you count the calories in the protein shake? Do you think you're counting the calories accurately?
Well, some random questions and possibilities. Good luck with it.