There are many reasons why our bodies don't "want" to lose weight:

hormones and thyroid issues
medications
proper nutrition (if you are lacking in something your body is "fighting" to keep systems running
allergies or food intolerances (sometimes weight gain is actual an immune response to some food intollerances).
sleep and stress - i call it either physically we are always hunting or running form the dinosaur. if this is the case, the stress hormones can prevent weight loss.
other health issues (some people mentioned diabetes) that our bodies are fighting with.

Working with a good holisitic dietician can help along with proper medical supervision.

Good luck!
Hannah