Physical limitation aside, maybe you are not benching optimally. When I do flat DB presses, my shoulders scream for mercy. With a flat barbell, I have no problems, once I sufficiently warmed up. The difference, I think, is that with a barbell, I’m better able to set up prior to lifting the weight. I find that if I retract, and hold my scapulae back, I can come bring the bar all the way to my chest without shoulder pain. If I’m not retracted, bringing the bar to my chest hurts my shoulder like holy hell. Retracting the scapulae not only gives you a stable base to push off, but brings your shoulders back with respect to your torso, allowing for a fuller range of motion.
Note: My shoulders are fucked. I have enough scar tissue, adhesions, and impingments for this entire board to share, but I can’t afford ART right now. Retracting the scapulae on the bench is just one of the little tricks I’ve picked up to continue lifting with my issues.
The ART website is http://www.activerelease.com