If you are doing flat benches, then you are feeling the soreness exactly where you should. Bench presses, while viewed as THE macho exercise, do not lead to overall pec development.

Try adding in cable cross-overs from both the high AND LOW positions. At the end of the movement, just don't douch you hands together, really cross them hard, and pause while you squeeze your pecs together. If your gym has a pec deck USE IT. While using the pec deck, make sure you bring your elbows together, and Squeeze! The above 3 exercises will build your inner pecs from all angles. Also, don't forget inclined presses to hit the upper chest.