Well, for what it's worth...
My chest lagged for years when I was younger. Really bad. Far behind the rest of my body. Training wise, what I found that seemed to work was to put that body part first trained in the week, and always use a continually changing routine/exercises in the 8-12 rep range with some intensity techniques like drop sets or what have you as well as some heavier sets in the 4-6 rep range thrown in. Then, on Friday, I would have another chest pump session, all between the 15-25 rep range, 3-4 exercises with 3-4 sets each with a total of 12 sets. Very similar to what I'm doing now. This, along with some preworkout insulin/GH on those days brought my chest from my weakest part to overpowering my shoulders and arms lol. I attribute the majority of the change to the training, but there's no doubt the insulin helped. But like most, I don't recommend the insulin unless you're extremely keen on your body's response and are willing to take a significant risk that could have potentially long lasting negative effects...think strange hypoglycemia problems like mine.
But the training is the big part. In my experience, you have to pay close attention to your connective tissue health when putting increased strain on a particular bodypart, especially for periods lasting longer than 4 weeks, to ensure you don't injure yourself. Otherwise you might end up overtraining that particular muscle group and go backwards with injuries instead of seeing improvement.
Just my two cents for what it's worth!