Inability to gain weight is caused by your body not assimilating nutrients properly.. or from what I've heard at least. If its true you should address the cause of that problem and you will stack on the muscle so long as you hit the weights hard ;D
Btw, hope your not taking whey protein or any other synthetic crap.. its bad for the liver and will mess you up down the line. Go with hemp protein.. it might sound weak but its the some of the most usable protein for the body.