NET Calories

I would say you aren't eating enough calories if you are burning 600+ calories a day. Maybe try eating back some of those calories.

Yup. Read the MFP FAQs...and use the tool you've got here. It works.

If you're not losing weight then you are eating more than you think you are or you are a statistical outlier whereby MFP's calorie goals wouldn't apply to you...usually it is the former.

Calorie counting is all about precision...there are numerous studies out there that indicate people underestimate their caloric intake by 20 - 30% or more just eyeballing portions, etc. Really, you need to be as precise as possible which means you need to use a food scale and weigh anything that has a serving size listed by weight and measure everything else. You also have to log consistently and log everything.

MFP is a NEAT method calculator which means your activity level is only supposed to include your day to day hum drum...it doesn't include any estimate of your exercise and thus your exercise calories. While other calculators do give you an estimate of those calories up front and then you get your cut, MFP doesn't do that. MFP only gives you credit for that exercise after the fact when you log it...thus you get exercise calories to eat back and you NET to your calorie goal. As an example...

Let's say someone using a TDEE calculator puts in exercise 3-5 hours per week and the TDEE calculator gives them a maintenance of 2300 calories. To lose 1 Lb per week that individual would eat a gross 1800 calories. Now let's say they use MFP and without exercise, MFP gives the individual 1500 calories NET to lose 1 Lb per week...but on average they burn around 300 calories per workout...so they would gross the 1800 calories just like the TDEE calculator. It's just two different ways of accounting for those exercise calories and making sure that activity is fueled and your deficit isn't too big.

On that note, I'd check your calorie burns as well. Burning 600 - 1200 calories per day in exercise is not necessary and you're probably not burning that much...people don't burn as much as they think they burn and if you're using a database for your numbers I can guarantee that they are inflated. It is very difficult to burn more than about 10 calories per minute and that is working pretty damned hard...so you aren't going to be able to sustain that level of exertion for long periods of time unless you are an endurance athlete. If you are overweight you can burn a bit more than the 10 calories per minute, but really...if your burns are super huge then you need to check those against multiple sources.

As I noted before, this is about precision and you have to be as precise as possible with both intake and output. Also note that weight loss is slow....when people tell me they haven't seen any "significant" weight loss after only 3 weeks I tend to think they don't really have realistic expectations of what their weight loss is going to look like and how truly slow the process is.

Its probably an error in calculating intake or burn calories or a mixture of both

http://www.acaloriecounter.com/blog/why-am-i-not-losing-weight/