Although muscular soreness is not a definitive marker for hypertrophy, our mentality as bodybuilders drives us to seek out pain and correlate it with a successful workout. Of course i am not speaking of the type of pain associated with injury, but the type of burning, searing pain that indicates to us that our muscles are being stressed. Even more uplifting to the serious trainee is awakening in the morning to stiffness and soreness in the muscle(s) that we targeted the day before. This provides a psychological boost and a feeling of accomplishment.
Since you say that your leg workouts are no longer making you sore i am going to assume that you have been performing more or less the same routine for too long a period of time. Although by simply changing your rep range, your exercise order, your rep tempo, etc, you can reintroduce soreness to your leg workout, i am going to take it a step further and suggest you change your entire leg program for the next 8 weeks. We are going to shock your mind and body with some unusual exercises, and make getting up and down the stairs the following day quite a chore!
Here is the routine:
-1 and a 1/4 leg extensions...3 x 8 (squeeze the weight to the top then bring it down only one quarter of the way before squeezing it right back to the top. Then bring it all the way down and repeat)
-hack sissy squat...3 x 12 (get in the hack machine and perform sissy squats by descending and bringing your butt off the pad, rising on the balls of the feet, and stretching your quads down and forward until the knees are in front of your toes and the quads are fully stretched. Push back to the top and repeat)
-bench step ups...2 x 15 (while holding a barbell on your back, put the working leg up on a flat bench and then step up onto the bench using only the strength of the working thigh. Complete all the reps with one leg before switching)
-single lying leg curl...2 x 8-10 (performed one leg at a time this simple exercise becomes quite intense)
-high stance leg press...2 x 10-12 (sit in a leg press machine and place your feet high enough on the platform so that your toes are off the top. Push through the heels and focus on using your hams)
-toes pointed hyperextension...2 x 15 (unlike standard hyperextensions, which work the lower back we will be targeting the hams. This is accomplished by pointing rather than flexing the foot and beginning each rep by consciously flexing your hamstrings. Only bring your torso to the parallel position and at the top further squeeze both the hams and lower glutes)
Have fun walking!