Your pec's have a droop or hang a little because the lower portion is over developed. Flat benches, but mostly dip's and declines, can cause this affect. The lower pec may atrophy a bit but still is influenced by inclines, fly's, etc to a lesser degree. Atrophy is a muscle/tissue wasting issue. Extreme loss of bwt weight is another way that a muscle could atrophy. For most guy's the upper pec's are a very hard area to develop because when doing presses (inclines or flat) the delts & triceps tend to come into greater play for most people. And genetics do have a role in pec muscle action-reaction, depth, width & shape.
Might try incline DB presses and fly's with the elbows held wide & a little higher than the front delts (shoulders) line . Use a pec deck, with it adjusted high enought where the elbows are higher than the shoulders and the elbows placed in the upper portion of the pads. There are some tricks to gripping & twisting the DB's when doing inclines which are beyond my means to explain, just to say that focus on keeping the little finger(s) higher than the thumbs in the DB grip. Touch both bells together firmly at the top position but do not lock out the elbows, keep the tension on throughout the press. Good Luck.
