Well, I actually have 2 thoughts on that.
One, a lot of people significantly overestimate the impact of their workouts and the number of calories they burn. As many people on FS mentioned, weight loss program is 80% percent about what you eat and only 20% about how much you burn in the gym, though I have no intention to underestimate the importance of good exercise program.
Two, it is also possible to overexercise. Your body will have same response to it as to any other stress (and in the same way as to starvation). You will start burning muscles for fuel and keep storing fat. If your program is too heavy on cardio and you do not spend enough time building muscles, this is a very likely scenario.
Only you can assess your work out program and decide if you overestimate the impact of your work outs or overdo it.
Anyway, if you do something for a long time and it does not work the way you want, try to change your approach and see if it works better. Good luck.
The first thing you lose on a diet is your sense of humor. ~ Author Unknown
It doesn’t matter what diet you follow… What matters is what makes you follow your diet. ~ Tom Venuto