Making sub-pages visible

I cannot seem to have visible sub-pages from my pages in the navigation bar. I have created a sub-page (child of the parent page) and it does exist, but I want it to be visible as a drop down option from the main parent page in the naigation bar. It is included in the blogroll, when I enable this in the sidebar, but for some reason not in the nagivation bar. What do I need to change to make this possible?