Keep a log of what you eat. Just put a pen and paper or make a note to enter it on your phone every night before you go to sleep. You should be able to easily recall what you had to eat that day.
Then after a week, spend the time and calculate our your calories and carb/fat/protein ratios. I guarantee you are nowhere near what you think you are eating. Trash eating to have high calories is THREE meals a day of Mcdonald's big mac supersized meal with a huge portion of the calories coming from the soda. If you drink water instead you will need to add two more big macs. So really five big macs a day plus fries to really have your calories up there for adding weight. There are far better bulking diets btw, just trying to get our mindset on what serious eating to add weight really means.
I was thin in high school and always wondered why. I started doing triathlons and learned what it meant to eat four to five thousand calories a day just to maintain weight. After my morning workout it took about 45 minutes just to get through breakfast and then I had to snack all day along with a few other big meals. This was just to maintain a body weight of 155 pounds. Of course, training 2-4 hours a day required the calories.