The old saying is that if you want to get big, you must eat big and lift big. The science behind this agrees, but it is more complicated than simply eating more. It does matter what you eat, how much you eat, when you eat, and how you exercise. It is not as simple as just getting on the SEE-food diet.
Recent research shows that even with a calorie surplus and resistance training, mainly gaining lean muscle mass is no easy feat.  See more at https://www.stewsmithfitness.com

The Science of Weight Gain
Be Patient: A recent study on weight gain for athletes and military personnel found that a 1-pound weight gain per week is a reasonable and optimal goal. To gain weight healthily, aim for a target gain of ½–1 lb. per week. Any faster, and you’re likely packing on fat rather than muscle. This process requires an additional 1,750 to 3,500 surplus calories at the end of the week or 250-500 extra calories a day.

You need good, healthy meals with extra portions of protein, carbs, and fat, plus snacks, to gain this kind of weight. Remember that the surplus required is in addition to the calories you burn just being alive (BMR – Basal Metabolic Rate) and the calories you burn through physical activity. Together, you will find your Total Daily Energy Expenditure (TDEE).  See the BMR Calculator and TDEE Calculator links for good ballpark figures of both. Reaching 250-500 calories above this will likely put you in the 4,000-calorie-per-day range, depending on your sex, body size, and level of physical activity. Ideally, these additional calories should come from nutrient-dense, energy-rich foods, such as peanut butter (or other nuts), dried fruit, and healthy liquid calories. Shakes and smoothies are easier to consume, but they do not satisfy you as much as solid food.

Train Smart: Muscle growth occurs across a spectrum of rep ranges from 12 to 20 reps, but training volume is the key factor. Mix it up, train to failure occasionally, and hit total rep targets (30–60 reps per exercise or three sets of 12-20 reps). To build more muscle, add 1-2 minutes of rest between sets. (Study on rest periods)

Check out latest podcast with Nick Barringer (PhD Nutrition) on overcoming the "Hard-Gainer" Life many of us had when starting out.