Discushuns, Welcome to SimplyDiscus!
Unfortunately without seeing the Discus when you got them or knowing more about what you feed and how you maintain them Its hard to say if they were poor quality when bought. Your main issues here though are the way you are keeping them. They Tank mates are not good for growing Discus. Your Discus just can't compete with them. You have only the strongest ones doing well. The others are stunted, and probably never will grow much or as well as they could have. The others just can't deal with it. The Substrate isn't helping either. Its just not easy to raise Discus in a tank as you have there. Best to raise them separate in a bare tank no tank mates as juveniles then you can put them in a substrate tank as adults. But honestly even then, The tank mates here are not good choices.. Too active.
al