1. You pick the right basic moves you can pile a lot of weight on, but then it is so heavy you need to use your shoulders and triceps more than your pecs to get the weight up. Add the typical turning the flat bench into a decline bench by raising your butt off the bench, and all you get is just more of the easily developed lower pecs, not high, round Arnold pecs.
2. Cable cross flyes are turned into a press and you're leaning all your body weight into the cables and then bounce the weight to the peak contraction. There is better way to do presses; with press movements.
3. Your workout schedule indirectly hits pecs several days around your pec workout. Maybe arms one day so you hit triceps, which assist pecs in press movements - the other day shoulders where your upper pecs will help a lot too. Solution: Make sure you do NOT train those muscle groups back to back with your pec day.
4. During presses, you recruit too much triceps by allowing your elbows to travel too close to your torso. Why does the body do this? You're stronger that way.
5. You have weak links that stop you from progressing. For instance weak rear shoulder muscles like infraspinatus and teres minor. These help the shoulders stay where they're supposed to be (pulled back). You might have a winged scapula, which means your shoulder blade is flaring out due to weak subscapularis or serratus anterior. A solution for the latter one is to do a lot of great pushups.