If you are aiming at gaining weight, it is important to calculate your current BMI and the target BMI. The target BMI should be in a healthy range. The problem here may be that you are eating too few calories to support your body’s basic functions. Aside indulging in moderate amounts of physical exercise it is also essential to maintain a healthy diet plan. It is important to know that a diet plan is not about cutting down or piling up calories; it’s about ‘healthy’ eating and balancing this with adequate amounts of physical exercises. When one takes fewer calories or works out more than required; the body sends a signal to the brain which in turn turns on the ‘stress hormones’ and the body switches into the ‘starvation mode’ and paradoxically one may notice no change in weight or an increased weight. One would need adequate amount of calories to support the basic metabolic functions and if you work out vigorously, your required calorie intake rises. A healthy diet should consist of adequate amounts of carbohydrates, proteins, fat and plenty of fruits and vegetables to supply vitamins and minerals. Also for consistent weight loss change of physical activities and diet plan works better following constant plan. You may try changing your exercise regimen. You may rotate between a gym, swimming, yoga, aerobics, jogging, cycling etc. Also keep a track of calorie intake and your work out. It might be useful to revise your diet chart as well. With gradual consistent efforts, you should be able to gain weight as desired in a healthy way. If still your weight does not seem to budge, I would suggest consulting a physician to rule out hormonal or endocrine abnormalities which may result in increased weight. If any abnormality is found, it would be treated appropriately.
Hope this helps.