How do you perform a critical finish in Soulcalibur IV (X-BOX 360)

I've knocked off all three areas of armor (top, mid, bottom), depleted the enemie's soul gauge & enrgy, and I still can't do a critical finish. I've been pushing the "L" bumper to no avail, all it does is perform a block. I've been trying to do this for hours in training mode, vs moe, and any or mode you can think of. What am I doing wrong?