Ever try using a power stretcher along with the knee kicker? In my experience, a knee kicker is good for smaller areas near the walls, but a power stretcher is way better for getting a tight pull from the center without those wrinkles. You have to hook it on the tack strip at the far wall and push the lever slowly. Just watch out for pulling it too tight, or you'll stretch it past the other side's tack strips and have to start over. It's a bit of a dance, going bit by bit down the room.