because exercise makes you stronger and the food might say that it does not have calories but it does. you should try weight watchers i lost two pounds in one 3 weeks. or take the special k Challenge.
if you aren't losing weight then you are still eating too many calories.
healthy or unhealthy calories are still calories.
you have to average 500cals. less a day than your body needs to lose 1lbs a week. so just cut down slightly on what you normally eat at each meal/snack and you will probably start losing.