Cannot gain any weight?

I am 18 years old, am 6 feet tall and weigh 65 kilograms. To try and put on weight, I have almost doubled my calorie intake for the last year, started heavy resistance training and have started taking dietary supplements such as m gainer. However, I have not put on a single kilo? How is this possible? What am I doing wrong and is there a problem here? It has been over a full year since I've started doing this. I feel extremely underweight and am constantly receiving comments, even to the point of being asked if I am anorexic.