You shouldn't neglect any part of your body. I mean, aside from all the various factors to health etc, you would look like a retard if you had massive arm muscles, chest muscles etc and flabby/weedy legs.
Weight gain is determined by how much food your body needs, how much it gets and certain genetic factors, such as your body's genetic type. Certain individuals, and ill assume with a great degree of accuracy, that you are one of thse individuals have an hard time gaining weight of any kind, be it fat or muscle mass.
If you want to gain, first of all, cut back on the cardio exercises, the ones that last a long time and burn loads of energy etc. This lowers your overall metabolism, meaning more energy is stored as fat. You can pack in the carbohydrates such as pasta, bread, rice etc.
You may just find that at your age, your still growing and the energy your taking in is being spent on your body's development, rather than buiding up muscle etc, and remember, if the energy you need is being spent on building your muscle, then your body will just burn the muscle to get what it needs, and thats less efficient than burning the energy from food, so your actually losing out.