Using a plastic wing-nut on a metal threaded shaft to drive the mechanism to "stretch" the width of the shoe is a guarantee of eventual failure.  The plastic wing-nut can be replaced with an 8 mm metal wing-nut for about 70 cents each.  Screw it onto the shaft "backwards" for the best function.  While replacing the wing-nut, consider using a larger washer, too.  I expect that the plastic piece at the end of the threaded rod will eventually fail, but I know of no easily available hardware that might work.  For about $2 this product could be greatly improved, but as it is, it was designed to fail.