I'm on my very firstest pair of socks & I'm shaping the heel. It seems when I slip the purl stitch it's much looser than when I slip the knit stitches and it tends to be a messy edge. Any advice on how to tighten this up? (I knit English, if that helps any.) I watched Amy's vid on messy edges/loose stitches, but all it recommends is to slip the first stitch to make it neater.