Subpage listing not working

I want to be able to show a listing of subpages on the parent page. I have tried several plugins and widgets but the listing always shows my main pages instead.

The page has one sub-page now (will have more in future) and want to use the listing to avoid having to edit a list of links. But the sub-page never shows up in the listing only the main pages.

Has anyone had this problem before? It doesn’t seem to matter which plugin or widget I try to use. They all produce the same result.