Hi there-I have had the same problem with chest aswell. No matter how much I was bench pressing the centre of my chest never grew. Even shifting to dumbells didnt help much.
Now I dont wish to be too controversial and there is nothing personal about it but I only beleive in part of totentanz statement. I think that some confusion comes through the fact that we are all different and our bodies respond differently. I have had training partners who seem to get away with only training bench press while having a massively built cleavage whereas I have always lacked in that area. It is correct that the entire chest muscle contracts during any kind of chest press and that there will be some growth throughout the entire muscle however I also think its a case of how the individuals muscle responds to training. Even though I am working the entire muscle during bench press the majority of the strain seems to go to my outer pec(I know technically there is no outer/inner but for the sake of describing different ares of the same muscle It needs to be used) whereas my training partner with good overall development seems to get strain more evenly through the entire muscle. Most people I know who believe that bench will work the entire chest equally are those gifted individuals who have never had to deal with a lagging inner chest.
What I have found works for me is variations of chins and shoulder press. My arms have also grown a lot with these variations.
-Chins-this is also excellent for bis/lats. Take an underhand chin grip with hands together and do a full chin up but once your arms get to shoulder height I bring your elbows together and pull them to your upper stomach-remember only go as far as you can while keeping the elbows together. I find this works the lower portion of the inner chest. Use pulldown machine if needed.
-Reverse grip shoulder press-this is also great for tris/anterior shoulder. The opposite movement to the above. Use a bar and start with a reverse grip, hands and elbows together and press the bar up to full extension. At shoulder height let your elbows come apart. I find this works the upper inner chest for me.
You can also try close bench but I found it worked mostly tris for me and pec dec/flye but I found this works more outer pec again.
My program looks like this-chest press, row, shoulder press, close grip chin, close reverse grip shoulder press, upright row, squat, crunches
Please remember this is what I found worked for me and you might need to try different things to work for you. Hope this helped