You're not eating enough. If you're not gaining fat on top of muscle you're not eating enough. If you don't have excess calories you will not grow. I use this + 1 scoop of regular protein powder. When I'm staring to gain too much fat I cut back on the mass gainer/excess calories and reset my diet.

You will probably pop a good 5-10lbs once you start using the mass gainer because your body is starving for excess calories to build with. This is also supplements on top of a full 3-meal a day diet.