I do push/pull days MWF, my pull days consisting of 9 sets of 4-8 reps compound back movements, and then 6-9 sets of 4-8 reps on bicep isolation. I do different arm positions for biceps (incline dumbbell curls, preacher curls, etc.). I focus a lot of the negative portion and generally go slow with around 30 seconds between sets and exercises.
I've noticed that whenever I do pull days, if my biceps get sore afterwards, it seems to always be the brachialis (sore near the joint and "outer" bicep area, not the much larger "two-headed" bicep ball), but not the long or short head of the brachii. My palms are always facing away from me for the entire movement on all biceps exercises.
I have no issue working different parts of my triceps, but for some reason, it seems as though my brachialis are always the part that gets hit and nothing else when doing biceps.
Maybe my brachialis is comparatively weak and therefore is the limiting factor? Maybe the brachialis must first be flexed before the brachii, and I'm unknowingly doing too limited of a range of motion to hit the brachii? Any other ideas?