can someone give me a good one, my legs just won't grow, i would like to think it's bad genetics because im good everywhere else, or maybe my routine sucks. anyway heres what i do, note that i cannot do free weight squats because it bothers the $hit out of my knee

smith machine squats 5 sets (1 warmup), 12/10/8/6
leg press 4 sets 10/8/6/4
lunges 2 sets of 10
3 sets of extensions, quads and hams

i will mix it up and do more sets of lunges, do a step machine and cut out the leg press, i am getting stronger, but no size. any thoughts, thanks