If you're trying to pack on some mass, you need to be sure you are eating enough food. Most people who think they have a hard time gaining mass are simply not eating enough food to support mass gain. You can use our calorie intake calculator to get an idea of your daily caloric burn, then you can begin by adding 500 to 1000 calories to that number.
Most times you will need to consume a 3000 calorie diet or a 4000 calorie diet in order to achieve some weight gain.
This sample 4000 calorie meal plan will have you eating six meals per day. If you can't eat six times per day, it is not a problem if you need to stuff these into 3 or 4 meals. Also, if you're currently eating somewhere around 2000 calories, you may want to gradually increase your caloric intake to prevent fat gain.
The only real way to pack on some mass is to eat more food. Our goal is to maximize muscle mass and minimize fat gain so you should be looking to gain around 1lb to 1.5lbs per week.