Puckering when changing stitches

I'm crocheting an afghan in a square. I need to lengthen it on two sides. I've been doing the back loops only. When I try to lengthen on one side going through two loops, it puckers just as if I had INCREASED the hook size. I tried decreasing the hook size from J to I, but it only helped a little. I'm trying to finish a Xmas project! Help!