Some people just don't have the genetics to have huge legs naturally. I had to bust my hump to get mass on my legs. Took me a couple cycles and training with a guy that trained at my gym that had monstrous legs. After a while they will start to grow. Even the toughest gainers will see results if they stick with it and eat right. Train,eat,rest. Just like trying to grow any other part of the body, those 3 things are key.