-Schedule a massive cheat meal the night before: Glycogen takes time to replenish; a cheat immediately pre workout isn't enough time. Also, schedule 8+ hours of sleep this night.
-Crush the weak bodypart every 4th day: I mean all out failure on every set, 20-30 sets. 3 is not enough days but 5 is too many; most of us are usually recovered sfter 5 days, so waiting 7 days between weak muscles is a waste of precious time to bring up weaknesses. Train smart and reasonable for everything else but go bananas on this day and beat yourself to a bloody pulp. Do battle with the weights 
-Eat more food the day of training: I up my carbs, protein, and fat 30% for every meal on this day. I like to stay lean year round and watch my diet closely, but every 4th day the goal is saturate my body with a nice surplus of good calories. Pre workout, I'll have my usual PWO meal but I'll also slam some simple sugars for a monstrous pump during the workout. Something like 40-50g sugar from whatever I want: candy, Gatorade, chocolate milk, etc.
-Skip Your cardio on this one day. The goal is to be anabolic, not catabolic. Fat burning is not the goal, save that for the other days of the week.
-Supplements/gear: take extra servings of your goodies on this day. Same philosophy as listed previously. If you take a pre workout drink, take double your normal dose. 30mgs Dbol? Take 60 on this day. I also take an extra multivitamin/mineral post workout on this day.
Post workout: No cardio today. stretch the muscle for a good 10 mins. Then go home and eat another big ass meal. The rest of the week is to diet properly, clean food and daily cardio, but on this day feed your body a caloric surplus. Lastly, get plenty of sleep this night, 8+ hours minimum.
This method has helped me greatly improve my Evan Centopanni Chest Syndrome.
Agree with trying to work in more volume. It's a volume game for sure.
I do a similar approach to improve my back. Trained it 4 days per week, 4 exercises, 3 sets per exercise, 15 reps per set, last set always to failure. It worked is all I can say. It worked enough that I'm still doing it (8 months later), and now my back is my best bodypart. It'll never be Yates' back, because I have a long torso, but I'm doing the best I can with what I have.
Day one: horizontal pulling (all kinds of rows...machines, dumbells, bbells, whatever...)
Day two: trap work (shrugs, high pulls, power cleans, power snatches)
Day three: lower back (reverse hypers, deads with bars or DBs, hypers, etc...)
Day four: vertical pulling (chins, pullups, pulldowns, 1 arm pulldowns, pullovers)
I would do this, and in between sets for back, super-set some other bodypart. Day five I worked arms on their own, because it's just real hard to do biceps supersetted with a back day. And legs were never a problem, so I trained them like a twink while I gassed out on back.
I'm waiting for my elbows to wear out or tear a biceps, but it hasn't happened. Knock on wood. I think it's because I use pretty light weights versus you monsters on here, and really try to focus on mind-muscle connection. Most guys with weak bodyparts will admit their mind-to-muscle connection sucks. So I always think that's a good place to start. If your back is weak, you should work to be able to (someday) just stand there, contract your lats, and make them cramp up. That's mind-to-muscle...not really scientific, just something important I think.
