You're doing reverse flys but not feeling your rear shoulders. Push the dumbbells out toward the wall instead of pulling them back, finish with elbows higher than your hands, and on the way down push your elbows outward rather than squeezing your shoulder blades.
If you are doing this exercise and no matter what, even if you have perfect form, you can never actually feel your shoulder muscles working, then try these 3 things out. It just might help. Number 1, instead of trying to go back behind your body with the weights, think about pushing the weight out to the sides of the wall. So instead of your hands finishing here, your hands will finish up here. Number 2, instead of your hands finishing above your elbow. You want to make sure your elbow is slightly above your hand. And number 3, when you're bringing the weight back, do not think about pinching your shoulder blades together. Instead, just think about pushing your elbow and pinky out to the wall, and you'll notice you'll feel the back of your shoulder a lot more.
