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.
6. The antagonist to the pecs is the big wonderful latissimus dorsi. I'm sure you've seen many gym people with ILS (imaginary lat syndrome) when there arms are flared out about 4 feet from the body trying to make that "big iguana" look, or what do you call that little lizard which makes itself extra large when threatened? Anyway, you need to keep the lats strong, but also flexible so they don't inhibit your pec strength.
7. Your mind is drifting away during workouts and you forget to feel the muscle. Don't just lie there looking up at the ceiling thinking about what to eat for dinner!
8. You neglect unilateral work. Dumbbells instead of barbells! You always have one dominant side, which will take over most of the work from the sissy side of your body. A great way to achieve strength increases is by prioritizing this before bilateral work.
9. You put the incline bench on too steep an angle, turning it into a front shoulder workout. Put it on a 30% degree angle instead of the conventional 45%.
10. You stick to machines due to a big ego. Yes you can use more weight on them than with free weights, but you also recruit fewer of the stabilizing muscles leading to imbalances. Machines are supplemental to free weights and should not be used as a replacement unless you end up at a hotel gym where the only option is a machine! In that case do tons of pushups and one arm pushups to balance it out.
Source:Sorry I really dont remember the source but I just found it inside my laptop,it might be retrieved from Muscle & Fitness if I am not mistaken. I just want to share it based on my courtesy on contributing something for the site to y'all so I hope this information found useful to you guys.