Bench Press is an awful movement for pecs, just look at powerlifters. Most have great front delts and thick trapz but rarely huge chests. Not to mention all the shoulder strain that flat bench can cause.

Most trainees that I see with a bad Chest usually have strong Delts and they end up using their shoulders on most Chest movements. Even Jay Cutler had this issue years ago.

You really need to arch your back big time, scapula down and retracted, chest high and only do the bottom 3/4 of the movement.

You would probably do really well with low-incline dumbell presses and decline bench press for your lower chest.