Try this:
I use dumbells on a bench where I can change angles.
Pick a weight you can get about 6 reps with and do a set on each notch from one notch from flat, all the way to a 45. So do a set move the bench, set- move bench up. I do three sets of those and call them torture sets. My chest has really gotten better up top and my strength is up. You definitely need to train upper chest first. I read that in a flex because if it is your weaker part of chest for muscle priority. Also one more thing maybe try do cable crossovers at several different angles.