Given the amount of exercise you're doing, 500-600 cals per day just sounds too low. I'm not big on the techy details, but your body may be stuck in "famine mode" where it's very reluctant to burn fat because it thinks it needs to hold on to it. MeoCompo says he's not big on that concept, but it makes sense to me.
As an example, my trainer did some calculations and reckoned that I need about 90g of protein per day, and I'm 41, male, 5'3 and 161 lb. So if I need 90, how much should an active 200lb man need? Not 60g, I'll wager.
As another example, the average healthy male is widely thought to need about 2000 calories per day. Your 600 calories is less than a third of that.
As perverse as it sounds, you could try eating more. I think you could even double your intake to 1200 calories without any adverse effects and that would still give you a substantial calorie deficit for your weight loss goals.
I stake no claim to any expertise, so feel free to ignore me.