biggest problem i see with people not being able to bring up their pecs is that they are so focused on how much weight they can use based on the old standby that you have to go heavy to get big, they lose the ability to be able to flex their pecs during the movement and end up relying on front delts and triceps to move the weight.
what i suggest that has worked for me, is to spend a few minutes flexing your pec muscles before you start to do your lifting. this will give you the ability to really feel the pec muscles working when lifting the weight. you must focus intensely on making sure you are squeezing your pec muscles at the top of the lift. sometimes it helps to back down in weight a little and focus on the flexing of the pec muscles during the lift then slowly add weight while making sure you are still maintaining a great flexation of the muscle.
i even will find myself flexing my pec muscles during the week almost every day just to be able to get a good feel for what it is like.
dumbells also work great for helping you to focus on the contraction of the pec muscle. what i do is when pushing up the dumbell, i rotate it so at the top of the movement, my palms are facing each other. this will help make the pecs contract during the movement.