[Genius]

As a lot of bodybuilders, I have a shoulder imbalance. My anterior deltoids are very developed. However, the posterior delts are lagging behind. Here's some of my information:

-25 years old
-175lbs, 5'9"
-6.5%-7% body fat
-I have been strength training for 9 years
-For around my first 5 years, I performed limited, if any, actual back isolation work. This was a mistake, I know. My strength coaches in highschool had us doing limited back work, so I followed suit.
-I've upped my back workouts in the past four years. My favorite pull exercises are chinups, pullups, isometric seated rows, and barbell rows.
-I can perform around 15 pull-ups (with strict form), and 20+ chin-ups (once again, with strict form)

I'm able to perform fairly well on my pull exercises. My question is, why do I still have the imbalance? I balance my workouts to include enough pull exercises to compliment the push exercises.

How can I correct this issue?

Thanks.