^ These sum up my thoughts.
Over the years I've tried different methods, read about many - but frequency and variation of intensity/volume seems to be the trick.
Forget too much focus on isolation movements and minimal training; heavy multi-joint movements for decent volume/frequency is where it's at with unresponsive legs. Any body part responds well to this (especially pulling movements), but legs seem to thrive on it.
Train them at least twice a week (even up to 3x/week! if you don't overload your lower back and you only do one major exercise for it). Pick around two major movements for each time;
E.g. Using a Mon-Fri-Wed type frequency:
Leg day 1
Squats (4-8 reps, 3-5 sets...less sets for high reps, and vice versa)
Lunges (10-12 reps, 2-3 sets)
Leg day 2
Deadlifts (3-5 reps, 3-5 sets)
Leg press (10-15 reps, 2-3 sets)
Something like that ^
Oh, and as has been mentioned, eat for it (if your bodyweight isn't going up monthly, you aren't gaining "squat")