Not losing weight

A couple of things to note...

The app will never go below 1200 calories, no matter what rate of loss you select. In fact, if it returns 1200 calories as your goal, thats usually a good indicator that you've chosen a rate of loss that is too high/aggressive for your current size. Meaning, if you're close to the healthy BMI range for your height, then 1200 calories is not going to cause you to lose 2 lbs a week anyway, since you have to have roughly a 1000 calorie deficit from your base maintenance calories for that to occur. And as I already mentioned, MFP doesn't go below 1200 calories, so if your maintenance is 1800 calories, for example, MFP will not give you 800 calories as a goal to get 2 lbs per week.
2 lb per week is usually only doable for those in the obese BMI range because they have a large enough range for the calorie deficit to work. For anyone smaller than that, they would lose that amount if they were undereating (not a good thing)

That said..

1 week is NOT enough time to gauge anything. Weight loss does not usually occur like clockwork, and lbs per week is an average over time, not a guaranteed weekly drop. Our bodies are not machines, they are living things and living things aren't always predictable, especially for women and their hormones. New exercise routines can cause the body to hold water weight for a couple of weeks, plus the body sometimes needs time to adjust to the calorie deficit. Usually when you're first starting out, it takes about 6 weeks before you can see what your weight loss trend truly is..