Is there a reason why I can't lose weight?

I've been trying to lose weight for over a year. It seems like it is literally impossible because if you do the math I shouldve been lost weight. I believe I eat 1500-2000 calories per day.but If u eat 2500 I won't gain or lose weight. I exercise everyday. Eat fruits everyday and I'm really trying to lose weight during summer 11. But it seems to impossible to the point I get frustrated because I basically tought myself everything about dieting and then I put it into action and I get NO positive results.I've actually gained 2 pounds over the last 2 months and I don't understand how. :( .. Btw I'm a 16 yr old boy 5'5 162 pounds