Several years back I had this same problem and the solution is very simple. When you wrap your hands, concentrate on your wrist. And then, above all, don't bend your wrist when you punch. I will guarantee you that you are doing this and here is an easy way to tell: when you land a right hand, which knuckles make contact? If it is not the middle knuckle and the index finger knuckle, you are almost certainly bending your wrist.