I had the same problem when i was losing weight last year. I was eating very carefully, and doing cardio avery day. It started coming off, then stopped. In the end i went and got some advice from a nutriotionist and a personal trainer.
It turned out that i had actually cut my diet back too much and my body had started "storing" rather than burning. So i actually had to start eating a little more! So beware of being too draconian.
On the other hand, Scarlett may be right, You may think you are eating healthily but pay close attention to portion size, as even a healthy meal wont help if you eat too much of it. Also, alot of snacks that advertise themselves as healthy have alot of sugars- even if low fat, and even "no added sugar" snacks can be bad if they are bursting with natural sugar. Check check and check the labels!
Also on the excersise side, the P.T got me to mix in some more resistance training as if you build muscle it burns the calories and fat better. Then when doing your cardio, mix up the intensity, do ten minutes high, then drop back to low then up to medium. Not only might your find you can keep going longer, but it also seems to help burn better.
Good Luck!