Most folks have a fairly even split of slow to fast twitch fiber composition, although a few trainers have significantly more of one than the other.
Either way, if you’ve been training with only a low-end rep scheme for a good while, then you may be neglecting your slow twitch fibers.
Try increasing your reps for a few months and see if that yields any kind of favorable results.
Also, you may wish to shorten inter-set rests to around the 60-90 sec. stretch, as this may release a bit more hGH.
I saw noticeable improvement a while back by doing that, alone.
A lot of guys prefer training with longer rests and lower reps - especially with chest - because it feeds their egos.
But, if you can put aside your ego to achieve different goals, you may be pleased with the results.