Some guy's who come to this site may want to write this down and paste it on their forehead less they forget: "No one has a perfect muscle balance on either side of their body". One side will be a little smaller than the other. One side will be dominate over the other side, that would also include strength, and it's usually not noticeable. Genentic has nothing to do with it. Attempting isolation movement (DB's, cables, pec decks, etc) can always improve the imbalance but the perfect match will not occur.
There is not a type of disorder that "makes" you miss some muscle. (dontknowit is a very good choice of a name) There is the pectorial major and the pectorial minor. Though they can be refered to with two division names they are one muscle unit that lives on your chest. Say hello to them because everyone has them.Yes women have them also under those stunning mounds known as mammary glands (thank you God).
Dips have a very direct influence on the lower portion of the pec's. May want to drop that exercise for awhile (including BB BP's) and focus on DB's, cables, pec decks, etc . These are not strict isolation exercises (nothing really is) but should bring up any lagging side of the chest. Decline presses will affect the low portion of the pec's also. Inclines (DB's) influence the upper pec's more. The pec deck tends to focus more on the actual action of the whole pec area because the point of stress is on the elbows. Good Luck.