I listened to a great podcast the other day about why we're not burning fat the way we'd like. You may know this but I'll mention it here again, fat is primarily disposed of via the breath. Calories, as well as excess carbs and proteins, are converted into triglycerides and stored in lipid droplets of adipocytes.
Here are a few reasons why you may not burn fat:
If you want to listen to the entire podcast, here is the source - click here.