Depends. What is your height? What is your bmi? If BMI is between 18.5-24.9, I'm not sure you need to gain any more weight. If your BMI is <18.5, you might need to eat some snacks between the meals to obtain adequate daily calories. Also how much exercise are you doing? Is your calories intake < calories output? If that's the case, you won't gain weight. See your doc to rule out other causes.