Do more squats and leg presses... Then eat a shit ton of food. Try and progress the weights on each set. Depending on your body type your rep range should fall somewhere in the 6 to 12 rep range. Push your self to failure on working sets. Don't just do back squats, you should mix in front squats, hack squats and sissy squats... Mix up your squat stances from narrow to shoulder width to sumo. Also don't forget to beat the crap out of the hams too not just your quads. Stiff legged deadlifts on a platform are great for that. Do super sets, giant sets, drop sets on occassion... Work hard and let your muscles repair then do it again. But most of all be patient. Don't be one of those guys that does three sets of squats per week and then says "i've tried everything but they just won't grow" and then go the rest of your life working just the glory muscles...