I find that everything becomes magnetic. The forceps, screwdrivers and everything sticks to everything. Eventually lady luck intervenes and success! Then you start all over again with the next valve. The manufacturers must have had a clever trick that they are not telling us about.
Seriusly though, the technique I have described does work. It seems to work best with inch suare pieces cut from the little pouches that some cat food comes in. Cats do have a use apart from catching mice.
Fred