How come im not losing weight?

I am a 14 year old boy, 5'4" and 155 pounds. I am very overweight, but work out faithfully. I always watch my calories, eat very healthy, and work out on the treadmill with HIIT for 20 minutes every 2nd day, and walk for 30 minutes other days. That PLUS gym in school, which is demanding. I have been doing this for many months, but how come im not losing my giant man ?, or flabby stomach? The scale changes every day from 146.5 to 147 to 145, why does it do that? How come i am not losing weight?