So, maybe you're wondering why? 'Why am I not gaining weight at all?' maybe it's in your Genes. Check if your whole family, even your relatives, if they're thin or not, if they are then it is in your genes, if not, then maybe it's in your Metabolism.
Metabolism tends to be fast especially with underweight people. They normally digest much faster than a normal person even if they're eating tons of food. Check out this Japanese woman who eats a lot of food and look at her. She's too petite, right?
If its not about your metabolism then you might have eating disorders,such as anorexia and bulimia. Anorexia is defined as the restriction of food intake due to the fear of becoming fat or overweight, while Bulimia is the tendency of a person to vomit their food after over-eating. These two diseases are a major cause of being underweight which can actually lead to cardiovascular, pulmonary and weakened bone conditions, and even death. You need the help of the professionals in order to fight these diseases so if you have it, kindly go and seek for medical help.
If it's not those, then maybe you're too stressed with what ever is happening is happening around you. If you are stressed your body will burn more calories so try to remember to not to stress out yourself a lot of times.
Wrong Choice of Food
Maybe it's with what you're eating. Salads and fish are healthy but these food aren't enough to pack your bones with the necessary fats and muscles. Remember to eat protein foods such as meat, and calorie-dense foods such as pasta, potatoes, rice, and bread.
Also try checking if you're too frequent in the gym, or you exercise too much as it can make you lose weight too. Exercising is a good thing but if you starve yourself before exercising then its not good.