I was always under the impression that over all pec development is pretty much unilateral. You have to develop all aspects of your perctoral muscle in order to fill out the inner portions of it. It just happens to be the last part to fill in or at least that's seems to be have been the case with me.
dumbell work and dips are as good as it gets for size development. just ucus on squeezing the weight up and back. fat will mask the true shape of your pecs.
Just an idea, when doing cable fly's, make sure to cross your arms so that your elbows at touching each other. Basically crossing your arms as far as possible. That will hit a range that you can't hit with bench or DB flys.
I'll generally do these at the end of a workout and i don't even go all the way out most of the time, i'll just focus on the very last part of the movement. Burns the piss out of my "inner" chest.
I think weighted dips while leaning forward stress the sternal attachment more than any other chest exercise. I could be wrong, but I do remember reading this.
Joe is right as usual about bodyfat hiding your chest development. Get your bf low enough and you'll see some inner chest striations.
the pecs are 1 muscle shaped like a fan. there is no upper, outer, inner. work various grips and angles. that is the way to full pec development. there is pec major and pec minor but that doesnt change the explanation i just gave.