you are likely going too far back then. Try shortening how far you pull back and see if that makes a difference. Use a smaller weight to start out and really force your chest muscles to pull the weight over. I could easily use the force in my back and arms to pull a lot of weight over, but that's not what I want to do. IMO, chest is the hardest muscle to isolate, so you have to keep your back AND shoulders flat, otherwise you are using more of another muscle than your chest.