For some people it is hormonal, I have PCOS which just fucks everything up. I wish for me it was as simple as Calories in VS Calories out. I've lost almost one hundred pounds so far and still going but I had to learn it's just not simple for everyone.
I'm not saying there aren't contributions, and I know I should have expanded my post... But calories in vs out is the base. Hormones can help or hinder that.