I'm a long time Lurker mainly but thought I'd give this a go...
K, Basically I've been training about 15 months going from about 161lbs to 191lbs although been in Hospital a couple of times
Staying around 12% from start to now.
My 1 Bench max is around 231lbs although chest and Shoulders are my weak points and grow slow and if I say the word Back mine can grow huge easily!
This is the workout I've been doing up until now and it seems to work for me until now. I take a week off usually every 10 weeks. I do roughly this in a week with tweaks here and there (Heavier weeks, lighter etc)
Squats - 3 Sets Reps 8 (If anyone knows how to get rid of the huge ass this gives you, please tell)
Leg Bench Press - 3 Sets Reps 8
Leg Extensions - 3 Sets Reps 8
Leg Curls - 3 Sets Reps 8
Calves - 3 Sets Reps 8
Change your training routine. You said you've been using this routin for 15 months. So it's time for a change. You have to stimulate your muscles on a new, another way. For example, change the reps, the exercises, the sets etc. I made the best experiences with changing the reps. So when I stopped gaining muscles I did 16 to 20 reps per set for about six weeks, and I started to grow again. Or you can change the intensity of training. Don't train until muscle failure for a few weeks.
I mean why should your muscles grow? Every week the same routine, the same exercises, the same food...just everything is the same. Just change something.