Make sure to keep the natural lordotic curve in your lower back: neither arching the back against the ball, nor rounding the back as you move down. Make sure you are standing far enough away from the wall so that your legs are at a 90=degree angle at the bottom.
If these still give you problems, try a towel wall squat instead: lean against a wall with a towel behind your back (not rolled up, just flast, as if you were lying on it on the floor). Slide down the wall into the squat, keeping your shoulder blades and butt against the wall/towel. You probably will be able to slide your fingers behind your lower back a bit (because of your natural lordosis), but you shouldn't be able to slide your whole hand back there (indicating an arched back). Also, make sure your feet are far enough forward so your knees don't go beyond your toes at the bottom position.
HTH!