Why won't one of the dustcaps on my wheel hub stay in place?

I can't see any way of getting it to stay in place. I need to know how to make it stay in place soon, because I will be having it built into a wheel soon - unless the problem is insolvable and I will have to replace it. It is on a fifty year old oil lubricated hub with the collar-type cones on the axel, if that makes any difference. I don't suppose that anyone can help, but it can't hurt to ask.