Posted by Mashboyellis

Don't just stuff your face with fatty, unhealthy food. Instead, try eating calorie dense foods such as nuts or seeds. These are high in protein and will help you gain weight, and you don't have to eat a lot of them.

If you can't eat big amounts at once, make a balanced meal plan. This will increase your metabolism and make you more hungry.
A 6 meal a day program would work in this case:

Small breakfast (Breakfast in important because it starts your metabolism for the day). If you like peanuts, peanut butter on toast is great. If you can't stomach it, I would recommend protein shakes. (Aim for 250-350 calories)

Morning snack, stick to something light, even a piece of fruit will do. (Aim for 50-100 calories).

Lunch: Have something basic like a sandwich or whatever you want. Again, keep it light, stay away from fibre. Fibre is hard to digest, and will stay in your stomach for longer, making you fuller for longer. That's why you're so full after that granola bar, people use them for losing weight because they're full of fibre. (Aim for 250-350 calories)

Afternoon snack: again, light. A good choice here would be a handful of nuts. (Which is around 160 calories alone)

Exercise is ideal between these two meals.

Dinner: This will be your biggest meal, but I suppose this depends on where you're from. Don't eat too late, or it will be hard to eat in the morning. (Aim for 500+ calories)

Evening snack: Have something that is easy to digest. Bananas are great before bed, they contain magnesium and potassium which help relax your muscles. (100-150 calories)

Gaining weight isn't about stuffing your face, that will just make you sick of food. If you want muscle, stick to a high protein, low carb diet if you can.
Despite this, I wouldn't worry about it too much. It may just be matter of moving around more and eating the right foods.