Hey way to stick with it and be on plan! That's awesome.
A few things that are the usual suspects:
1. Underestimating your portion size. How do you know you are eating 1200? If you are eating what you think is 1200 and not losing, the most likely scenario (it's happened to all of us!) is we are measuring or eyeballing our food incorrectly. Weighing is much, much more accurate.
2. Your body loses in whooshes, not in increments. That's how I, and many, lose. I'll see the same number on the scale for 2 weeks, then drop 1.5 lbs overnight. It's quite common. You just have to stick with it.
3. This isn't really a "suspect" but a caveat- I would NOT try to assume how many calories you are burning on the treadmill. If you are walking (even on an incline) for 45-60 minutes, that's awesome, but you are likely burning less than 500 calories. Runners generally burn 600 calories an hour; walking is much less.
This is not a reason that you shouldn't be losing- any exercise contributes to your deficit, so I'm not suggesting you up the exercise or lower your food intake. Your exercise is AWESOME- it's more than the vast majority of folks get in each week. It's just a caveat when you are doing mental calculations, developing expectations on your rate of weight loss, etc. Read more here on the sticky thread in the Exercise forum:
http://www.3fatchicks.com/forum/exer...g-methods.html
So when you think of how much exercise you are doing and how you aren't losing, I am prompted to remind you what many have reminded me- weight loss really is 80% diet and 20% exercise! Much more dependent on the former and not the latter.
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It looks like you are at the beginning of your journey, so it's unlikely you are stalling just yet. I would give it another week or so and really make sure you are getting your portion sizes correct. Measuring cups get you close, but only a food scale is really accurate
good luck!