I've done a few layers where I've used a wire like this (I usually don't bother with a wire) and honestly, it's never seemed to really work as intended. The trees have still managed to push callus past the wire. Maybe that means I haven't applied it tight enough? Anyway, I've been battling a similar situation with an attempted ground layer on a maple, for a couple of years. Each season I've had to recut the layering site because the callus just grows right past it and roots don't form. I don't really have room to extend the cut further down so I just keep trying. This year I added a stronger rooting hormone gel so fingers crossed.