How do I ask without being a bitch?

So, I just got a roommate in August. She has yet to clean and the one time she did "say" she cleaned all she did was spray stuff in the air. I tried to just do it and not worry about it. Cleaning doesnt take that long. Its a really small house and all she would have to do is the floor and kitchen. It take 1/2 tops. We got a new roommate a month ago and we sat down and made a cleaning schedule where we just each clean the house on the weekend one at a time and just go in a row - week 1 - me, week 2 - roommate 1, week 3 - roommate 2 and then we start all over again. Well last weekend was her turn. She never cleaned. So I said hey if you wanna just do it monday or tuesday thats fine and we will keep going in rotation. She never cleaned. So the next weekend went by and I finally did it. Now im just sick her not helping. Its super easy. We all go to college, we all work, and we all have a life. Why is it that she cant pitch in?
What can I do to make get this ball rolling?