Hello and welcome! At 15 you use so many kcals just to support the normal growth phase for that age. On top of that you are burning an very high number of calories due to the strenuous exercise that you engage in. You did not mention how or what you eat to give me an idea except that you eat "healthy" You need to eat adquate protein for your current weight which is 108 grams of lean protein such as fish, white meat of chicken or turkey, vegetable protein, only thin cuts of beef, low fat milk, low fat cheese, low fat cottage cheese, greek yogurt snacks, trail mix snacks but don't go for the sugar it will make you sluggish and slow you down. Basically, son, you are not eating enough calories from the correct sources at the right times is my intuitive answer based on the limited information I received. Add those snacks; eat 6x per day so carry trailmix with you or something quick; eat larger portions of your healthy foods making certain you get 108 g daily (figure 8g/glass of milk; 7 g/oz of protein - meat; greek yogurt 10-14g); 1 0z cheese 7g) ); keep hydrated to digest your kcals and tune your body - cannot increase protein without increasing fluid. Do you understand now your approach to weight gain?