No matter what i do i can't put on weight. i am underweight but i always eat a balanced diet. i eat like a horse but can't gain weight. please help.

Gaining weight: Foods to help with healthy weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, Avocadoes, granola, trail mix, dried fruits, extra virgin olive oil, canola oil, protein shakes, protein bars, lean meats, poultry, fish, Smoothies (with added silken tofu or powdered milk), Greek yogurt (full –fat), pasta/ bagels & use of juice rather than soda. Some people add whey.