[Hey Ranga. You're just not eating enough. You may think you're eating a lot, but you really aren't. I used to think I couldn't gain weight no matter what I ate too, but it turns out it all boils down to counting calories and eating more than you need. Track what you put in your mouth for a day and you'll see.

Here are some tips:

If you're trying to lose the skinny look, your target weight should be one kg for every cm of your height above 1 meter. For example, I'm 1.69m. That makes my target weight 69kg.

To gain weight, the general rule is to eat 20 calories per pound of body weight per day. Example: I weighed 121lbs (55kg), so to gain weight I need to eat 20 x 121 = 2420 calories a day. That gain is a bit slow though, so I ramped it up to about 3100 to reach my target weight faster.

You want to get bigger, not fatter, right? Then you need to complement the eating with gym workouts 3 times a week. Preferably get on a strength training program because more strength means more muscle, and more muscle means more fat-burning capacity.

It's a lot of info to take in at once, but I think this is a good start. Feel free to ask me if you need more info, I'll be glad to share my experience.]