If your diet is good than it could also be because of hyperthyroidism. It prevents weight from gaining. Take 5 meals a day, 3 major and 2 minor Try to consume a good portion of food from each food group. Take nutritious smoothies and milk shakes twice daily. Don't rely only on chapatti and curries that we normally make at home. Try to supplement your food with different food groups for example, if you're making soup, add a whole 1 egg in your bowl and cook it so that you get the nutritients of 1 whole egg. Sometimes when timings aren't fixed, it causes problem too. I cannot say any thing unless i get your detailed history. There might be something that you're doing wrong or you have been overlooking.
I would like to add one more thing that its not a single fruit or food but a combination of them that helps in weight gain. You can come and meet me in my clinic; you can also get online consultation if you want Drop me a message https://www.facebook.com/yumnachattha