It does sound like a strange discrepancy. Can you get a picture of her with you holding her? Obviously, we can't guess her accurate weight from a picture, but I dare say we could tell the difference between a 600g snake and a 1500g snake.
AND...have you ever had her checked for parasites? It's possible that she may have internal parasites and that would keep her from gaining weight even if she is well fed. And if you got her from a "breeder" who would turn around and give you that kind of attitude at this point in time...it'd be a safe bet to say they might sell imported animals (most likely to have internal parasites) as captive bred.