Helped me in the past
I would usaually answer this by telling folks to focus on the big, basic, movements like squats, leg presses, ect. Keep the form tight and controlled and just focus on getting stronger in these movements week after week. In my own situation years ago before ever competing I did seem to reach a point where my lower quads just weren't progressing with the rest of the upper/middle quads and was creating what I call a Turnip or raddish thigh. I had access to the University of Mich, and Eastern Mich Univ sports medicine labs while going through their Physiology program. We would hook up Electrodes to various areas of the muscle in question. These electrodes were part of an imaging machine that measured loads, motor unit efficientcy, and muscle/nerve stimulation so we could adjust our form and foot positioning in order to see how those changes effected diff areas of the quad. In the old days before this technology,they say that guys like Arnold and Vince Gorinda would test an exercise's focus by doing only that one exercise for 20 sets to purposely overtrain it and see what regions of the muscle were in the most pain in the days to come. Sorry so long winded with this response but I really don't paraphrase very well and wanted to be thorough. Basic findings supported what Mr. Boom and other PM members opinions were. For the lower and lateral sweep of the quad: focus on feet close or even touching together and positioned toward the lower area of the Leg Press and Hack Squat platform. Also make sure the kness travel over the toes while descending. Hit them deep in the hole but only 3/4 of the way up before starting the next rep down. For the teardrop (vastus medialis) you want to use a slightly narrower than shoulder width stance with the toes pointed slightly outwad. Again, make sure the knees travel over the same plane as the big toe and use the 3/4 reps for the compound movements. Although I am not a fan of leg extentions, I did see very good progress by starting the workout in a pre-ex manner. Leg extension with proper warm-up, then 2 balls out sets of aprox 12-15 full reps to failure then try matching that number with immediate 1/2 reps. Only do the lower 1/2 of the movement once reaching failure even if you are only getting a 6 inch movement (just bite down and match that #!). Then move on to your multi-joint movements. I did have to lighten the weights initially but still gained muscle in the desire areas.
Damn, that was long winded! Sry, i really need to work on that!
This obviously won't work for everyone but I had great success in making my lower quads look like what a bodybuilder's should. Good luck