I had the same problem. When doing pressing movements your shoulders are taking up most of the load since that's your strongest muscle group in that compound movement. You have to isolate the pecs. Cable cross-overs aren't really that good because you can't use a lot of weight and it still taps into the shoulders and triceps. Use a good pec deck machine and drive with your elbows using full range movements and squeeze at the full contraction position. Do force reps and negatives if you can. Get that full stretch. When you can do anymore reps burn it to hell with partials. Keep going until you can't get another fraction of an inch of movement. Then while the muscle is pumped to the gills stretch back as far as you can still using the same weight and hold that full stretch for several minutes until you are about to cry. Then try to stretch back even more by trying to push your chest forward while still keeping your elbows back. Then, while your pecs are trashed, immediately go to a pressing movement.