Hello dear, surely will help you.
See,There are many reason of being underweight.
1. Endocrine disturbances
2. Genetic
3. Vitamins and minerals deficiency
4. Psychological problem
5.IBS
6. Eating disorder
7.
Liver dysfunction
And many more
So my opinion is to contact us directly as you are in Bangalore, so that we can find the cause of not gaining weight , and according ly we will suggest you diet plan which will not only increase your appetite and weight but also cure your deficiencies.
Next Steps
For more information and to contact me directly visit https://www.facebook.com/pg/doctorplanmydiet/about/
Thanks with regards
Dt.Ankita