I've gotten several PM's over the last couple of years from people (women in particular) complaining about how they were eating low calorie diets (1200 or less) and doing all kinds of cardio and yet still were not meeting their weight-loss goals. My response to them has been this...

Either you have...

#1 failed to calculate your calories (and therefore deficit) correctly

-OR-

#2 damaged your metabolism with dieting/cardio  

Reason #1 is self explanitory. Some people are actually eating more than they think they are. If you need to be precise... get a food scale and measure everything!

Reason #2, on the other hand, is a bit more difficult to comprehend. I have, however, found a video that I think does a good job in explaining this phenomenon. If you are having this issue then you need to see it! It's worth the watch!

http://www.youtube.com/watch?v=QHHzie6XRGk&feature=player_embedded