^^ I still go as heavy as possible on all my exercises to a minimum of 6reps. But the most recent change to my upper routine and likey the cause for upper chest would likely be standing strict push presses and leaning forward dips (till failure at the end of workout 4 sets). For dips I'll try to hold the stretch for 5 seconds each rep (Isometric contraction). Personally started to feel like flys were overrated and cut them out for now.
Last upper day I did push press, isometric pull ups, flat bench, barbell rows, incline db press, cable facepulls, isometric dips.