I think the Milk powder that u r referring to is "Weight Gain"...
Anyway, from what I know... sometimes forcing ur self to gain weight wont work that well if your body is not really that type.. I mean for some people their body is very fattening.... a slight change in their diet could have a huge difference shown within weeks!! I guess this has something got to with your genes!! Cuz if your family members are bit "skinny type" then its obvious that you derived it from them!! But if that's not the case just give some time.... You'll get gain enough weight when your right age comes!!
And importantly don't just do whatever you think is right to gain weight... I think you better consult a doctor first, and get professional opinion 1st!!