Maximum weight loss heart rate zone?

I have been taking cardio kickboxing and boxing at the LA Fitness for 6 months and have not lost weight. I drastically changed my diet by cutting fast food to once a weeks and eating non stop fruits and veggies an lost a pound or two. I take those classes 5 times a week and are vigorous because according to my heart rate moniter I peak at 90% max heart rate. Do I really have to do 50-60% max heart rate to lose weight when I burn more calories at 70-85% max heart rate?!?!?