holy shit!
You have 21 sets focused on quads and 4 sets (stiff leg deads) hitting hams. Is this a typical leg day?
I see a few things that could be your culprit:
-Overtraining the quads
-Training the muscle(s) with an inneficient order of exercises
-Undertraining the hams
-Keeping the same routine which leads to stagnation
Ok, here's what I would recommend:
-split quads and hams up into two different workouts so you can focus on them better.
-do at least 10 sets, but no more than 15 sets for any muscle. I recommend 10 for most people, but pros talk about doing up to 15 sets.
-Mix up your reps. If you usually do 8-10, try doing 12-15 instead for a while. Every now and then throw in a killer set to 20 reps to break the routine.
-For each muscle, pick 1 or 2 exercises from each category of the basic types of exercises:
1) full range/compound movements, typically doesn't keep resistance on the muscle or muscle group in either the flexed or stretched position. Typically a compound movement utilyzing auxillary muscle groups to do the motion. Do this with maximum poundages.
2) stretch position movements, keeps resistance on the muscle in a full stretched position.
3) flexed position movements, keeps resistance on the muscle in the full flexed position.
-Do compound movements first, then stretch, and finish off with flex. I always do this, unless I am going for pre-exhaustion for a particular group.
For Quads:
Your squats, diamond bar squats and leg presses are all compound exercises. Hack squats, sissy squats and lunges are stretch position exercises. Leg extensions are flex position exercises.
On quad day try:
>Squats... 5 sets 12-15 reps (use the first two sets at warmups)
>Lunges...4 sets, hold dumbells in your hands and lunge across the gym 4x
>Leg Extensions...4 sets 12-15 reps, hold each rep for a 2 second squeeze at the top
For Hams:
Lying and seated leg curls are the best option for full range. Stiff leg deads are for stretch and single-leg machine leg curls are good for flex position.
On Ham day try:
>Lying leg curl...5 sets 12-15 reps (use the first two sets at warmups)
>Stiff leg Deads...4 sets 12-15 reps, think about your hams and ass during the motion and lift the weight by squeezing your ass/legs like you are trying to fuck the mirror (in other words, don't even think about your back and arms except to keep them in good form)
>Single leg machine leg curls...4 sets 12-15 reps, hold each rep for a 2 second squeeze at the top
****Work every set like it's your last....give everything you have and go to failure! More work IS NOT as good as less work done MORE INTENSELY!*******
Give that a try and be sure to mix up the exercises on each workout.