I dont know how true this statement is but ive been told the chest is the hardest muscle group to develop. Was told by a friend this, not sure if its true but thats how im feeling at the moment.

Im really struggling to see progession in my inner chest in particular. My current Chest workouts include:

Bench - 4 Sets 6-8 reps
Incline Bench - 4 sets 6-8 reps
Decline Flys - 4 sets 6-8 reps
Regular Flys/Dips - 4 set 6-8 reps (usually alternate this exercise everyweek)

Ive even started to include inner chest exercises during my arm days, doing weighted dips in between benches and close grip bench press. With all this though, this area of muscle is where im seeing the least growth. Any tips on improving the chest?? as its the look and muscle group that i love to workout and see mass gain in the most.