Eat a combination of junky AND healthy foods. Get some good quality steak/beef in you, and some veggies also.
I'm not exactly sure the reason why, but I knew a few people who basically lived off only junk foods, and were skinny. One of them has recently had to start eating healthier becuase of other heal problems, and has started putting on weight- just from eating healthier.
I wish I knew an explanation for this, but I don't. Just incorporate some healthy foods, and see how you go.